Musnad Ahmad 27:1695

'Isam ibn Khalid narrated to us, Shu'ayb ibn Abi Hamzah and Abu al-Yaman narrated to us, they said: Shu'ayb ibn Abi Hamzah informed us, from al-Zuhri, Salim ibn 'Abd Allah narrated to me, from 'Abd Allah ibn 'Umar: "That the Messenger of Allah (may Allah bless him and grant him peace) used to perform voluntary prayer (tasbih) while he was on the back of his riding beast, not caring where its face was directed, and he would gesture with his head, a gesture." And Ibn 'Umar used to do that.