Musnad Ahmad 27:413

Yazid narrated to us, Isma'il informed us, from Abu Hanzalah, who said: I asked Ibn 'Umar about the prayer while traveling. He said: "The prayer while traveling is two rak'ahs." He said: "We are secure; we do not fear anyone." He said: "The Sunnah of the Prophet, may Allah bless him and grant him peace."