Musannaf Ibn Abi Shayba 16:4

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Muhammad ibn Ishaq, from Abdullah ibn Abi Bakr, from Muhammad ibn Ali ibn al-Husayn, from Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) performed the *aqiqah* for al-Hasan with a sheep. He said: "O Fatimah, shave his head and give in charity the weight of his hair in silver." So they weighed it, and its weight was a dirham or part of a dirham.