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كتاب الاستسقاء
15  
Invoking Allah for Rain (Istisqaa)
(22)
Chapter: The raising of both hands by the Imam during Istisqa' while invoking Allah for rain
(22)
باب رَفْعِ الإِمَامِ يَدَهُ فِي الاِسْتِسْقَاءِ
Sahih al-Bukhari 1031

Narrated Anas bin Malik:

The Prophet (ﷺ) never raised his hands for any invocation except for that of Istisqa' and he used to raise them so much that the whiteness of his armpits became visible. (Note: It may be that Anas did not see the Prophet (ﷺ) raising his hands, but it is narrated that the Prophet (ﷺ) used to raise his hands for invocations other than Istisqa. See Hadith No. 807 & 808 and also see Hadith No. 612, Vol. 5).

حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنَا يَحْيَى، وَابْنُ أَبِي عَدِيٍّ، عَنْ سَعِيدٍ، عَنْ قَتَادَةَ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ كَانَ النَّبِيُّ صلى الله عليه وسلم لاَ يَرْفَعُ يَدَيْهِ فِي شَىْءٍ مِنْ دُعَائِهِ إِلاَّ فِي الاِسْتِسْقَاءِ، وَإِنَّهُ يَرْفَعُ حَتَّى يُرَى بَيَاضُ إِبْطَيْهِ‏.‏
Reference : Sahih al-Bukhari 1031
In-book reference : Book 15, Hadith 26
USC-MSA web (English) reference : Vol. 2, Book 17, Hadith 141
  (deprecated numbering scheme)