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كتاب الاستسقاء
15  
Invoking Allah for Rain (Istisqaa)
(18)
Chapter: The Istisqa' prayer consists of two Rak'a
(18)
باب صَلاَةِ الاِسْتِسْقَاءِ رَكْعَتَيْنِ
Sahih al-Bukhari 1026

Narrated `Abbad bin Tamim from his uncle who said:

"The Prophet (ﷺ) invoked Allah for rain and offered a two rak`at prayer and he put his cloak inside out."

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ عَبْدِ اللَّهِ بْنِ أَبِي بَكْرٍ، عَنْ عَبَّادِ بْنِ تَمِيمٍ، عَنْ عَمِّهِ، أَنَّ النَّبِيَّ صلى الله عليه وسلم اسْتَسْقَى فَصَلَّى رَكْعَتَيْنِ، وَقَلَبَ رِدَاءَهُ‏.‏
Reference : Sahih al-Bukhari 1026
In-book reference : Book 15, Hadith 21
USC-MSA web (English) reference : Vol. 2, Book 17, Hadith 138
  (deprecated numbering scheme)