(1) باب صَلاَةِ الْمُسَافِرِ
Narrated Aisha, Ummul Mu'minin:
The prayer was prescribed as consisting of two rak'ahs both when one was resident and when travelling. The prayer while travelling was left according to the original prescription and the prayer of one who was resident was enhanced.
Reference: Sunan Abi Dawud 1198
In-book reference: Book 4, Hadith 1
English translation: Book 4, Hadith 1194