Sahih Muslim 959
صحيح مسلم#959SahihAnas b. Malik reported. The Messenger of Allah (ﷺ) said:Perform bowing and prostration well. By Allah, I see you even if you are behind me, or he said: (I see you) behind my back when you bow or prostrate
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 4:121