Ziyad bin Jubayr (may Allah have mercy on him) says that Ibn Umar (may Allah be pleased with them both) passed by a man who was slaughtering his camel after making it kneel. He said: Make it stand, tied (and then slaughter it); this is the Sunnah of your Prophet (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 10215