Musannaf Ibn Abi Shayba 16:29

Abu Bakr narrated to us, he said: Affan narrated to us, he said: Abd al-Warith narrated to us, he said: Ata’ ibn al-Sa’ib narrated to us, from Muharib ibn Dithar, from Ibn Umar, who said regarding the *aqiqah*: "It is slaughtered for him on the seventh day, and his head is shaved, and the weight of his hair is given in silver as charity, and his head is smeared with blood."