Musnad Ahmad 27:1734

Al-Fadl ibn Dukayn narrated to us, Malik—meaning Ibn Mughawwil—narrated to us, from Abu Hanzalah, who said: I asked Ibn Umar about the prayer of travel? He said: "Two rak'ahs." I said: "What about the saying of Allah, Blessed and Exalted is He: {If you fear} [Al-Baqarah: 239], while we are in a state of security?" He said: "The Sunnah of the Messenger of Allah, may Allah bless him and grant him peace," or he said: "Such is the Sunnah of the Messenger of Allah, may Allah bless him and grant him peace."