Yahya related to me from Malik from Hisham ibn Urwa that hisfather used to kill his sacrificial animals while they were standing.Malik said, "No-one is permitted to shave his head until hehas killed his sacrificial animal, and no-one must sacrifice beforedawn on the day of sacrifice. The things that should be done on theday of sacrifice are slaughtering, donning clothes, grooming the bodygenerally (at-tafath) and shaving the head, and none of this may bedone before the day of sacrifice."
USC-MSA web (English) reference: Book 20, Hadith 192
Arabic reference: Book 20, Hadith 891