Musnad Ahmad 26:52

'Affan narrated to us, Hammad ibn Salamah narrated to us, from 'Asim, with his chain of narration, he said: Abu Bakr came to him with a hollowed-out rock, and he milked into it. He drank, and Abu Bakr drank, and I drank. He said: Then I came to him after that and said, "Teach me from this Quran." He said, "You are a taught boy." He said: So I took seventy surahs from his mouth.