Musnad Abu Ya'la al-Mawsili 162:20

Muhammad ibn Abdullah ibn Numayr narrated to us, Hafs narrated to us, from Ja'far, from his father, from Ali ibn al-Husayn, from Ibn Abbas, from al-Fadl ibn Abbas, who said: I was riding behind the Prophet, may Allah bless him and grant him peace. He threw the Jamrat al-Aqabah with seven pebbles, and he said "Allahu Akbar" with every pebble.