Musnad Ahmad 26:317

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