Musnad Ahmad 27:1372

'Affan narrated to us, Wuhayb narrated to us, Musa ibn 'Uqbah narrated to us, Salim narrated to me, that 'Abdullah used to pray at night and perform the Witr prayer while riding on his camel, not caring which direction it faced. He said: And I have seen Salim doing that, and Nafi' informed me, on the authority of 'Abdullah, that he used to attribute that to the Prophet, may Allah bless him and grant him peace.