Musnad Ahmad 26:296

Waki’ narrated to us, from Sufyan, from Abu Ishaq, from Khumayr ibn Malik, from Abdullah, who said: "I recited seventy surahs from the mouth of the Messenger of Allah, may Allah bless him and grant him peace." And indeed, Zayd ibn Thabit had a lock of hair, and he was among those who wrote [the revelation].