The Prophet`s Companions are the Witnesses of the Religion

Hazrat Abdullah bin Masood (may Allah be pleased with him) said: "If a person wants to follow someone, he should follow someone who has passed away because a living person can fall into temptation at any time." He further stated, "The group of companions (Sahabah) is worthy of being followed, as they were extremely pious and knowledgeable."

The companions (may Allah be pleased with them) were not only the first narrators of the Quran and Sunnah but also of the circumstances of that era. The Quran invited the world to faith in this manner: "ان آمنوا بمثل ما اٰمنتم بہ فقد اھتدوا وان تولوا فانما ھم فی شقاق" If they believe as you (companions of the Prophet) have believed, then they are rightly guided; but if they turn away, then they are in severance. In this way, the Quran has made the companions the standard of religion and truth.

Another aspect is that the foundation of religion is the Quran, the Sunnah of the Prophet (peace be upon him), and the group of companions (may Allah be pleased with them). From the fact that:

•  the Quran provides the principles and regulations, 
• the Sunnah of the Prophet is its practical form, 
• and the group of companions represents its collective and social form. 

When Ummul Momineen Hazrat Aisha (may Allah be pleased with her) was asked, she said: "کان خلقہ القرآن" His character was the Quran. I translate this as: what is theoretical in the Quran is practical in the Sunnah.

Some time ago, someone raised the question that while we respect the companions, there should be a "definition" of who a companion is. I responded that this has already been established and cannot be reopened. 

The companions are those whom the Ummah has considered companions for fourteen hundred years and whom "محدثین" (the scholars of Hadith) have classified as companions in the books of "اسماء الرجال" (study of the biographies of narrators). Among them are "اہل البیت" (the prophet's household), "مہاجرون" (migrants to Medina), "الانصار" (supporters of the migrants), and also "والذین اتبعوھم باحسان" those who converted to Islam after the conquest of Makkah. There is a classification among them, but there is no difference in their being a proof and an ideal.

The saying of Abu Zur'ah Razi (may Allah have mercy on him): "اذا رايت الرجل ينتقص احداً من اصحاب رسول اللہ صلی اللہ علیہ وسلم" is in this very context that "If you see a man criticizing any of the companions of the Messenger of Allah (peace be upon him), then know that he is undermining the witnesses of the religion".

https://zahidrashdi.org/5608