Musnad Abu Ya'la al-Mawsili 117:152

Qatan ibn Nusayr narrated to us, Ja'far narrated to us, Thabit narrated from Anas, who said: Rain fell upon us while we were with the Messenger of Allah (may Allah bless him and grant him peace). The Messenger of Allah (may Allah bless him and grant him peace) uncovered a part of himself [to let the rain touch him]. He said: "Indeed, it has recently come from its Lord."