Assalamu 'aleikum, I have a Salafi friend who told me that knowledge can only be accepted from ulama (scholars) who have learnt this knowledge through a chain of ulama that goes back to the companions of the Prophet, sallallahu 'alayhi wa sallam. In other words, you need to find out who the Sheikh has studied with, and who that one has studied with, etc. If not, you should not accept the sheikh's teachings. I have never found any proof for this. Can you please clarify this matter?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger. 

There is no doubt that it is very important that the seeker of knowledge gets his knowledge from a Shaykh who would show him the correct path. Indeed, the scholars are lighthouses of guidance that help people understand the texts of the Book of Allah and the Sunnah of the Prophet . Verily, the Salaf (righteous predecessors) have warned against taking knowledge directly from books to an extent that one of them said: “Whoever has his book as his teacher, his mistakes are more than his rectitude.”

As regards the statement that it is a condition when taking knowledge from a Shaykh to know the chain of Shaykhs from whom he had received knowledge, then we do not know any scholar who said this. Rather, this statement implies that those who deserve that we take knowledge from them are too few. Therefore, this statement is very inappropriate.

Finally, we warn against the danger of insisting on hardship in matters of religion. Indeed, it is confirmed that the Prophet said: “Ruined are al-Mutanatti’oon (i.e. those who go to the extremes).” [Muslim]

An-Nawawi said when interpreting this Hadeeth and clarifying the meaning of al-Mutanatti’oon: “It means those who exaggerate and who exceed the limits in their statements and actions."

Allah Knows best.