Consensus of other Sahaba are required for Islamic law if Hadith is narrated by a single Sahaba:

بِسۡمِ ٱللَّهِ ٱلرَّحۡمَٰنِ ٱلرَّحِيمِ

Some excerpts from at-Tawassul wal-Waseela by Ibn Tayyimiyah (Rah)

We can't decide a  law relying on the sole narration of Sahabi. If the meaning of the Hadith conflicts with the understanding of the Sahabi, we should give importance to the text of the Hadith rather than the explanation of Sahabi.

Regarding Enjoining acts of Worship and its prohibitions, if a single Sahabi is narrating about Prophet (saw), and if the other sahaba didn't agreed to its narration and if the other narrations are in agreement with other sahabis opinion, then laws are framed on basis on the opinion of majority of Sahaba. A Muslim should not act on the lone opinion of the Sahabi and should stick to the majority. But regarding that lone opinion, the Sahabi would get the benefit of Ijtihad.  But the masai'il should be of worth for ijtihad.

There are various proofs for not deducing Islamic laws on lone opinions.  Umar (Ra) used to wash his eye pits during Wudhoo. He would take new water for wiping the ears. Abu Huraira (RA) used to wash his hands until he reached the upper arm and said, "whoever wants his face to Shine on the Hereafter should wash up to upper arm (Sahih Muslim 246b). Abu Huraira(Ra) used to wash his necks and would say this is the place where one would be chained. 

Though Umar(RA)  and Abu Huraira (RA) said  these are Sunnah,  majority of the scholars have differed in this.  They say other than these two sahabis none of them did wudhu like this. In the Hadiths regarding Prophets Wudhu , many hadiths are found in Bukhari and Muslim and there is no Narration available that Prophet (Saw) fetched new water for wiping the ears or Prophet (saw) washed upper arm and above the ankles and Prophet (saw) wiped on his neck.

There is no narration from Prophet (Saw) like what Abu Huraira(RA) said i.e. one should wash up to upper arm for the face to Shine in Hereafter. They are words attributed to Abu Huraira (RA) in some of the Hadith. He (saw) said,

"They (my followers) will come with bright faces and white limbs because of Wudhu". Prophet (saw) used to wash his arms and feet up to ankle and Prophet(saw) words "Khurrath" belongs to face. In order to make the face brighter, one should wash his face and not washing hands beyond arms or washing beyond ankles. In Arabic "hujlath"  is the term used for whiteness of Hands and feet and not Khurrath. Abu Hurairah(RA) sais "He who can afford among you, let him increase the brightness of his forehead and that of hands and legs" and people who heard it would have mistaken for washing arms for increasing the brightness of the face.

The brightness of the Face in the Hereafter would be given only to the face , because face is a defined organ. One should wash his face fully. Only then Wudhu would be valid.. Washing the forehead for increasing the whiteness of the face is an wrong act. Prophet (saw) did not say that the forehead or head would be made white. It’s not considered as Sunnah to wash the Hands or feet beyond the prescribed boundary. "Musla" is the term used for washing the upper arms.

Let’s taken another example. Ibn Umar (RA) used to follow the Prophet(Saw) exactly. So much so that he would even would love to walk in the footsteps in Prophet(saw) walked. Ibn Umar(RA) would break his journey and stay in

places wherever Prophet(saw) broke his journey and stayed. He used to stand and perform wudhu where Prophet(saw) would do wudhu. He would pour the remaining water on the plants and trees where Prophet(saw) poured the water.

Ibn Umar(RA) would do many such acts.

Only few of the scholars recognize this kind of following. They consider it as Mustahab. But majority of the scholars do not consider this as Mustahab.

Some of the great Sahabis like Abu Bakr (RA), Umar(RA), Uthman (RA), Ali(RA), Muadh ibn Jabal (RA), Ibn Masud(RA) and other great Sahabis never did like what Ibn Umar (RA) did.  If they thoughts as good deed, they would

have done that. Because they fully followed Prophet (Saw) and they completely changed their life styles.

Following of Prophet (saw) is to follow him in his actions what he voluntarily did, we have to do it exactly . If the act is for the worship, we to have to do the worship in the same way. If certain places or time has been allocated for certain act, we too have to allocate for the same. He(saw) Circumambulated Kabah for Tawaf. He (saw) kissed the Black stone,  prayed behind Maqam Ibrahim. He loved to pray behind a particular pillar in Masjidul Nabavi. He loved to  climb Safa and Marwa and supplicate. During Haj, would supplicate in Muzdalifah and Arafah. All of these were done with an intention to do. We also have to do the same and we have to fully follow him in these acts.

But if the act has been done without intention, like getting down in certain place during journey. Praying if it’s time for prayer time etc. All these acts are not done intentionally. One may get tired during journey and may break the journey because of that. If these actions are done without intention, why do we need to break our journey in particular places where Prophet(saw) broke his journey. If we allocate such place and time for such acts it wouldn't be like following

Prophet(saw). Umar Ibn Katthab scolded about these acts and called it as Innovation. 

It was narrated that al-Ma’roor ibn Suwayd (may Allaah have mercy on him) said: We went out with ‘Umar ibn al-Khattaab, and we passed by a mosque on the way. The people hastened to pray there, and ‘Umar said, “What is the

matter with them?” They said, “This is a mosque in which the Messenger of Allaah (peace and blessings of Allaah be upon him) prayed.” ‘Umar said, “O people, those who came before you were doomed because they followed such practices, until they established places of worship in such locations. If a time for prayer comes when you are there, then pray, and if it is not the time for prayer, then move on.” Narrated by Ibn Abi Shaybah in al-Musannaf, 7550.

When ‘Umar ibn al-Khattaab heard that there were people who came to the tree under which the Prophet (peace and blessings of Allaah be upon him) had received the bay’ah (oath of allegiance), he ordered that it be cut down.

Narrated by Ibn Abi Shaybah in al-Musannaf, 7545.

Because Prophet(saw) did not intentionally choose the place and time for these acts. But it’s true that Prophet (saw) got down at those places and prayed and this is known very well to Umar (RA). Yet he forbid as such innovation and condemned it as it was like resembling  Jews and Christians in acts of worship. If someone does that then it is outwardly following Prophet (Saw) but by principles following Jews and Christians.

This is the reason why scholars opinionated on jalsat al-istiraahah, i.e. resting after sujood while  getting  up for Qiyam for second Rakath. Prophet (saw) did sit before getting up. But what is the purpose of sitting? Did Prophet (saw) did that with intention or did he do it because of some requirement. Scholars who considered this as intended act, made it as sunnah while the scholars who considered acts as unintended, didn't make it as Sunnah.

Similar difference of opinions can be seen among various masaail. For example, Prophet (Saw) got down in Wadi Muhassab while returning in Haj from Mina. Scholars have difference of opinion on whether Prophet's (Saw) action was intended or unintended.  Abu Talha(RA) allowed eating Hail stones  fasting. We have to understand in the same way about Ibn `Umar(RA) touching the seat of the Prophet's minbar. This is same as saying Takbir on Eid Night by Ibn Abbas (RA) in Basara and Amr Bin Harisa (RA) in Kufa. These are not Mustahab because majority of the Sahaba (RA) didn't act on it and Prophet (Saw) didn't command as Shariiya law.

A Sahabi called Hudayfa (RA) permitted to eat for person fasting in day time  even if the sun has arisen. Umar (RA) and Ibn Umar(RA) had said that it is Kharahath(disliked) to use perfumed things before beginning Tawaf. Some Sahaba (RA) totally prohibited fasting during travelling. Salman (RA) considered Saliva as Najas (impure).

Also Ibn Umar(RA) used to forbid marrying People of Book. Also Ibn Umar(RA) and Ibn Umar(RA) used to say that a person with Major impurity should not do Tayammum. Ibn Umar(RA) and some Sahaba(RA) said "If a person  dies wearing Ihram, his Ihram becomes Baatil (nullified). Laws applicable to normal person would applicable to him". Muadth and Muawiyya (RA) said "A Muslim could become Inheritor of Kaafir's property. Ali (RA) and Ibn Abbas

(RA) said if a husband of pregnant women dies , she should in iddah(waiting period) twice the period. Ibn Abbas(RA) and others said the woman whose husband dies should confine to house.

Similarly,  Sahabis have expressed many opinions regarding many such Mas'alas. If a  Sahabi had expressed an opinion and others have differed with this Sahabi, then the laws are not formed on lone opinion. It is the Ijtihad of the Lone narrator and there is reward for even incorrect Ijtihad.  Madhab Imam and other scholars are of the same opinion.  No one has the right to declare on their own as sunna, wajib , haraam etc.  Some of the things forbidden by  Khulafa Rashideen are also to be understood that it was based on Prophet(saw) order.  The only person to declare as Wajib, Mustahab, Haraam, Makrooh is none other than Prophet (saw).