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كتاب الاستسقاء
15  
Invoking Allah for Rain (Istisqaa)
(16)
Chapter: To recite aloud while offering the prayer of Istisqa'
(16)
باب الْجَهْرِ بِالْقِرَاءَةِ فِي الاِسْتِسْقَاءِ
Sahih al-Bukhari 1024

Narrated `Abbad bin Tamim:

from his uncle who said, "The Prophet (ﷺ) went out to invoke Allah for rain. He faced the Qibla invoking Allah. He turned over his cloak (inside out) and then offered two rak`at and recited the Qur'an aloud in them."

حَدَّثَنَا أَبُو نُعَيْمٍ، حَدَّثَنَا ابْنُ أَبِي ذِئْبٍ، عَنِ الزُّهْرِيِّ، عَنْ عَبَّادِ بْنِ تَمِيمٍ، عَنْ عَمِّهِ، قَالَ خَرَجَ النَّبِيُّ صلى الله عليه وسلم يَسْتَسْقِي فَتَوَجَّهَ إِلَى الْقِبْلَةِ يَدْعُو، وَحَوَّلَ رِدَاءَهُ، ثُمَّ صَلَّى رَكْعَتَيْنِ جَهَرَ فِيهِمَا بِالْقِرَاءَةِ‏.‏
Reference : Sahih al-Bukhari 1024
In-book reference : Book 15, Hadith 19
USC-MSA web (English) reference : Vol. 2, Book 17, Hadith 136
  (deprecated numbering scheme)