مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3244
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn ‘Abbas (رضي الله عنه) said: The Messenger of Allah (ﷺ) prayed two rak'ahs when he travelled, and when he was not travelling he prayed four, Ibn 'Abbas said: Whoever prays four rak'ahs when travelling is like the one who prays two rak'ahs when not travelling. And Ibn ‘Abbas (رضي الله عنه) said. He only shortened the prayer once, when The Messenger of Allah (ﷺ) prayed two rak'ahs and the people prayed one rak'ah each (in two groups, as in the fear prayer)
Musnad Ahmad 3268lts isnad is da'eef; Humaid bin Ali is da'eef, it is a repea