Sayyiduna Anas bin Malik (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) ordered that the heads of Hasan and Husayn (may Allah be pleased with them both), the two sons of Sayyiduna Ali bin Abi Talib (may Allah be pleased with him), be shaved on their seventh day; then he gave in charity silver equal to the weight of their hair, and he did not slaughter any other animal.
It has also been said that it is from Yahya bin Sa'id, from Amrah, from Aishah (may Allah be pleased with her), but that is not preserved (mahfuz).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19271