Bukhari

Narrated Abu Wail: Hudhaifa said, "I saw a person not performing his bowing and prostrations perfectly. When he completed the prayer, I told him that he had not prayed." I think that Hudhaifa added (i.e. said to the man), "Had you died, you would have died on a tradition other than that of the Prophet Muhammad."

This was narrated by Abu Humayd, the Companion (may Allah be pleased with him), from the Prophet (peace and blessings of Allah be upon him).