6. The Book of Prayer - Travellers - كتاب صلاة المسافرين وقصرها
1) Chapter: The Prayer Of Travellers And Shortening Of It
'A'isha, the wife of the Messenger of Allah (ﷺ), said Allah prescribed the prayer as two rak'ahs, then it was completed (to four rak'ahs) at the place of residence, but was retained in the same position in journey as it was first made obligatory.