Mu'tamirnarrated from his father, from Barakah, from Bashir binNahik, fromAbu Hurairah:
"The Prophet (ﷺ) supplicated for rain(raising hishands) until I saw or one could see the whiteness of hisarmpits." (RU)
English reference (RU): Vol. 1, Book 5, Hadith 1271 (RU)
Arabic reference (RU): Book 5, Hadith 1330 (RU)