Musnad Ahmad 27:12

Hushaym narrated to us, Yunus informed us, Ziyad ibn Jubayr informed me, he said: I was with Ibn Umar at Mina, and he passed by a man while he was slaughtering a camel, and it was kneeling. He said: "Send it standing, bound, [according to] the Sunnah of Muhammad, may Allah bless him and grant him peace."