Musnad Ahmad 26:264

Rawh narrated to us, Sa'id narrated to us, from 'Abd al-Salam, from Hammad, from Ibrahim, from 'Alqama, from Ibn Mas'ud, that the Messenger of Allah (peace and blessings of Allah be upon him) used to fast while traveling, and he would break his fast, and he would pray two rak'ahs, not abandoning them—he says: He would not add to them—meaning: the obligatory prayer.