Musannaf Ibn Abi Shayba 3:1451

Hushaym narrated to us, from Ibn ‘Awn, from Mujahid, who said: We asked him, "Did Ibn ‘Umar perform voluntary prayers during travel?" He said, "No." I said, "What about the two rak'ahs before the dawn prayer?" He said, "I never saw him abandon those two, neither in travel nor at home."