Musnad Ahmad 27:1131

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Yunus ibn 'Ubayd, from Ziyad ibn Jubayr, who said: I saw Ibn 'Umar pass by a man who had made his mount kneel and intended to slaughter it. He said: "Standing, bound; the Sunnah of the Messenger of Allah, may Allah bless him and grant him peace."