Sahih Muslim 1571
صحيح مسلم#1571SahihA'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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 6:2