Sahih al-Bukhari 791
صحيح البخاري#791SahihNarrated Zaid bin Wahb:Hudhaifa saw a person who was not performing the bowing and prostration perfectly. He said to him, "You have not prayed and if you should die you would die on a religion other than that of Muhammad
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 10:186