Sahih Muslim Book 7, Hadith Number 3032.

Chapter : The camel is to be sacrificed in a standing posture and fettered.

Ziyad b. Jubair reported that Ibn ‘Umar came upon a person who was slaughtering (sacrificing) his camel and had made him kneel down. So he told him to make it stand up festered (and then sacrifice it) according to the Sunnah of the Holy Prophet (may peace be upon him).