It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him): "The Messenger of Allah (peace and blessings be upon him) ordered that the hair of the heads of Sayyiduna Hasan and Sayyiduna Husayn (may Allah be pleased with them both), the sons of Sayyiduna Ali bin Abi Talib (may Allah be pleased with him), be shaved on the seventh day after their birth. So their heads were shaved, and then he (peace and blessings be upon him) gave in charity silver equal to the weight of that hair, and he (peace and blessings be upon him) did not find any animal to slaughter (for the aqiqah at that time)."
Hadith Referenceالمعجم الأوسط / باب الألف / 127
Hadith Takhrij«أخرجه البيهقي فى «السنن الكبرى» برقم: 19332، والبزار فى «مسنده» برقم: 6199، 6200، والطبراني فى «الكبير» برقم: 2575»