Musnad Ahmad 27:1477

Ishaq narrated to us, he said: I asked Malik about a man who performs Witr while he is riding. He said: Abu Bakr ibn Umar ibn Abd al-Rahman ibn Abdullah ibn Umar ibn al-Khattab informed me, from Sa'id ibn Yasar, from Ibn Umar, "That the Messenger of Allah, peace and blessings of Allah be upon him, performed Witr while he was riding."