كتاب صلاة الاستسقاء‏
9  
The Book of Prayer - Rain
(1)
Chapter: Raising The Hands In Supplication When Praying For Rain
(1)
باب رَفْعِ الْيَدَيْنِ بِالدُّعَاءِ فِي الاِسْتِسْقَاءِ ‏
Sahih Muslim 895 c

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.

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 ‏.‏
Reference : Sahih Muslim 895c
In-book reference : Book 9, Hadith 7
USC-MSA web (English) reference : Book 4, Hadith 1953
  (deprecated numbering scheme)