Musannaf Ibn Abi Shayba 3:2253

Ali bin Mushir narrated to us, from Dawud, from Abu Uthman, from Mutarrif bin Abdullah bin al-Shikhkhir, who said: I came to al-Sham, and there I was with a man praying, bowing, and prostrating, and he did not separate [the units of prayer]. I said, "If only I sat until I guided this old man." He said: So I sat. When he finished the prayer, I said to him, "O servant of Allah, did you depart upon an even number or an odd number?" He said: "I have been spared that." I said: "And who spared you?" He said: "The noble scribes [angels]. I have not performed a single prostration except that Allah raised me a degree by it and removed a sin from me by it." I said: "Who are you, O servant of Allah?" He said: "Abu Dharr." I said: "May Mutarrif’s mother be bereaved of him! He is teaching Abu Dharr the Sunnah!" When I came to the house of Ka’b, it was said to me, "He has asked about you." When I met him, I mentioned to him the matter of Abu Dharr and what he said to me. He said to me the same as what he said.