Maliks Muwatta Book 20, Hadith Number 192.
Section : How to Make the Sacrifice.
Yahya related to me from Malik from Hisham ibn Urwa that his father used to kill his sacrificial animals while they were standing.
Malik said, “No-one is permitted to shave his head until he has killed his sacrificial animal, and no-one must sacrifice before dawn on the day of sacrifice. The things that should be done on the day of sacrifice are slaughtering, donning clothes, grooming the body generally (at-tafath) and shaving the head, and none of this may be done before the day of sacrifice.”