Musnad Ahmad 799:6

Muhammad ibn Ja’far narrated to us, Sa’id and Yazid narrated to us, they said: Sa’id informed us, and Bahz narrated to us, Hammam narrated to us, from Qatadah, from al-Hasan, from Samurah ibn Jundub, from the Prophet, may Allah bless him and grant him peace, that he said: "Every boy is held in pledge by his ‘aqiqah (sacrifice), which is slaughtered for him on his seventh day." And Bahz said in his narration: "And blood is smeared [on his head], and he is named on it, and [his hair] is shaved." Yazid said: "His head."