Musnad Ahmad 25:520

Yahya ibn Abi Bukayr narrated to us, Ibrahim ibn Tahman narrated to us, Khalid al-Hadhdha’ narrated to me, from Ikrimah, from Ibn Abbas, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) performed Tawaf upon his camel. Whenever he came to the Corner (the Black Stone), he gestured toward it and said 'Allahu Akbar'."