Sahih Muslim , Book 9, Hadith 5 (In-Book) ⓘ
Anas reported: I saw the Messenger of Allah raising his hands (high enough) in supplication (for rain) that the whiteness of his armpits became visible.
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا يَحْيَى بْنُ أَبِي بُكَيْرٍ، عَنْ شُعْبَةَ، عَنْ ثَابِتٍ، عَنْ أَنَسٍ، قَالَ رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَرْفَعُ يَدَيْهِ فِي الدُّعَاءِ حَتَّى يُرَى بَيَاضُ إِبْطَيْهِ .
also referenced under: Hadith 895 (Reference) & Book 4, Hadith 1951 (USC-MSA)