Musannaf Ibn Abi Shayba 21:530

'Abdullah ibn Idris narrated to us, from Layth, from Muhammad ibn 'Abd al-Rahman ibn Yazid, from his father, who said: I accompanied 'Abdullah [ibn Mas'ud], and he threw seven pebbles, saying "Allahu Akbar" with every pebble. He kept to the bottom of the valley until he finished. Then he said: "O Allah, make it an accepted Hajj and a forgiven sin." Then he said: "This is how I saw the one upon whom Surah al-Baqarah was revealed act."