Musnad Ahmad 937:29

'Affan narrated to us, Hammad ibn Salamah narrated to us, from 'Ali ibn Zayd, from Mutarrif, who said: I sat with a group of the Quraysh, and a man came and began to pray; he would bow and prostrate, then stand up, then bow and prostrate, not sitting. I said: "By Allah, I do not see that this man knows whether he is finishing on an even or an odd number." They said: "Will you not stand up to him and speak to him?" He said: So I stood up and said: "O servant of Allah, I do not see that you know whether you are finishing on an even or an odd number?" He said: "But Allah knows. I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'Whoever prostrates a prostration to Allah, Allah writes for him a good deed for it, removes a sin from him for it, and raises him a degree for it.'" I said: "Who are you?" He said: "Abu Dharr." So I returned to my companions and said: "May Allah reward you with evil as companions; you ordered me to teach a man from the companions of the Messenger of Allah (peace and blessings of Allah be upon him)."