Muwatta Malik 330
موطأ مالك#330Sahih(with regards to above) Malik said, "I believe that was during rain." Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to join the prayer along with the amirs if they joined maghrib and isha in the rain
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Salim al-HilaliMauquf Sahih
In-book reference: 9:5