Musnad Abu Ya'la al-Mawsili 152:241

Abu Khaythama narrated to us, Rawh ibn 'Ubadah narrated to us, Sa'id narrated to us, from 'Abd al-Salam, from Hammad, from Ibrahim, from 'Alqama, from Ibn Mas'ud: That the Prophet, 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 and not leave them. He says: He would not add anything to them, meaning the obligatory prayer.