Sahih Muslim 1944
صحيح مسلم#1944SahihIbn Umar reported that the Messenger of Allah (ﷺ) observed prayer in danger on some day (in this way):a group stood with him (the Holy Prophet) (for prayer) and the other group stood In front of the enemy. Then those who were with (him) observed one rak'ah of prayer and they went back and the others came and they observed one rak'ah (with him). Then both the groups completed one rak'ah each. Ibn Umar said: When there is greater danger, then observe prayer even on the ride or with the help of gestures in a standing posture
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: 6:372