Musnad Ahmad 26:511

Jarir narrated to us, from Layth, from Muhammad ibn Abd al-Rahman ibn Yazid, from his father, who said: I was with Abdullah until he reached the Jamrat al-Aqabah. He said: "Hand me some stones." He said: So I handed him seven stones. He said to me: "Take the rein of the she-camel." He said: Then he returned to it and threw the seven pebbles at it from the bottom of the valley while he was mounted, saying "Allahu Akbar" with every pebble. And he said: "O Allah, make it an accepted Hajj and a forgiven sin." Then he said: "Here is where the one upon whom Surah al-Baqarah was revealed used to stand."