Musnad Ahmad 27:1776

Isma'il narrated to us, Yunus informed us, from Ziyad ibn Jubayr, who said: I saw Ibn Umar come upon a man who had made his sacrificial camel kneel down in order to slaughter it in Mina. He said: "Send it standing, bound, [according to] the Sunnah of Muhammad, may Allah bless him and grant him peace."