Musnad Ahmad 27:1761

Sulayman ibn Dawud narrated to us, Abd al-Rahman ibn Abi al-Zinad narrated to us, from Musa ibn Uqbah, from Salim ibn Abd Allah, that Abd Allah ibn Umar used to pray his night prayer while traveling, and he would perform the Witr prayer while riding upon his camel, not caring which direction his camel faced. And he attributed that to the Prophet, may Allah bless him and grant him peace. Musa said: And I saw Salim doing that.