Maliks Muwatta Book 23, Hadith Number 3.
Section : Animals Desirable as Sacrifices.
Yahya related to me from Malik from Nafi that one time Abdullah ibn Umar wanted to sacrifice an animal at Madina. Nafi said, “He told me to buy him an excellent horned ram, then to sacrifice it on the Day of Sacrifice in the place where the people prayed.” Nafi continued, “I did so and when the ram was sacrificed, it was carried to Abdullah ibn Umar who shaved his head. He was ill, and did not attend the Id with the people.” Nafi added, “Abdullah ibn Umar used to say, ‘Shaving the head is not obligatory for someone who sacrifices an animal.’ Ibn Umar would do so however.”