It was narrated that Ibn 'Abbas (رضي الله عنه) said:

The Messenger of Allah (ﷺ) prayed two rak'ahs when travelling and four rak'ahs when not travelling. Ibn 'Abbas (رضي الله عنه) said: Whoever prays four rak'ahs when travelling is like the one who prays two rak'ahs when not travelling. Ibn 'Abbas (رضي الله عنه) said: Prayer was not shortened except 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].