مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3790
Musnad of Abdullah bin Mas'ood
It was narrated from Ibn Mas'ood (رضي الله عنه) that The Messenger of Allah (ﷺ) used to fast when he was travelling (sometimes) and he would not fast (sometimes), and he would pray two rak'ahs only, i.e. he did not add to them - i.e., the obligatory prayers.
Musnad Ahmad 3813lts isnad is da'eef jiddan (very weak)]