Sahih Muslim 895
Sahih Muslim · Book 9: The Book of Prayer - Rain · Raising The Hands In Supplication When Praying For Rain
حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى ، حَدَّثَنَا ابْنُ أَبِي عَدِيٍّ ، وَعَبْدُ الأَعْلَى ، عَنْ سَعِيدٍ ، عَنْ قَتَادَةَ ، عَنْ أَنَسٍ ، أَنَّ نَبِيَّ اللَّهِ صلى الله عليه وسلم كَانَ لاَ يَرْفَعُ يَدَيْهِ فِي شَىْءٍ مِنْ دُعَائِهِ إِلاَّ فِي الاِسْتِسْقَاءِ حَتَّى يُرَى بَيَاضُ إِبْطَيْهِ . غَيْرَ أَنَّ عَبْدَ الأَعْلَى قَالَ يُرَى بَيَاضُ إِبْطِهِ أَوْ بَيَاضُ إِبْطَيْهِ .
Anas reported that the Messenger of Allah (ﷺ) was not accustomed to raice his hands in any supplication he made except when praying for rain. (He would then raise [his hands] high enough) that the whiteness of his armpits became visible. 'Abd al-A'la said that (he was in doubt whether it was) the whiteness of his armpit or armpits.