Musnad Ahmad 27:1141

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Jabir, [who said]: I heard Salim ibn ‘Abdullah narrating from Ibn ‘Umar, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) would not pray during travel except two rak‘ahs, except that he would perform the night prayer (tahajjud) at night." He said: And Ibn ‘Umar would not pray during travel except two rak‘ahs, except that he would perform the night prayer (tahajjud) at night. Jabir said: So I said to Salim: "Did they both perform the witr prayer?" He said: "Yes."