Imam Malik's Muwatta, Chapter: Hajj

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."