Sunan An-Nasa'i - سنن النسائي
17. The Book of Praying for Rain (Al-Istisqa') - كتاب الاستسقاء
5) Chapter: The imam turning his back to the people when supplicating during prayers for rain
It was narrated from Abbad bin Tamim that :
His paternal uncle had told him that he went out with the Messenger of Allah (ﷺ) to pray for rain. He turned his rida' around, and turned his back to the people, then he prayed two rak'ahs and recited loudly.
5) باب تَحْوِيلِ الإِمَامِ ظَهْرَهُ إِلَى النَّاسِ عِنْدَ الدُّعَاءِ فِي الاِسْتِسْقَاءِ
أَخْبَرَنِي عَمْرُو بْنُ عُثْمَانَ، قَالَ حَدَّثَنَا الْوَلِيدُ، عَنِ ابْنِ أَبِي ذِئْبٍ، عَنِ الزُّهْرِيِّ، عَنْ عَبَّادِ بْنِ تَمِيمٍ، أَنَّ عَمَّهُ، حَدَّثَهُ أَنَّهُ، خَرَجَ مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم يَسْتَسْقِي فَحَوَّلَ رِدَاءَهُ وَحَوَّلَ لِلنَّاسِ ظَهْرَهُ وَدَعَا ثُمَّ صَلَّى رَكْعَتَيْنِ فَقَرَأَ فَجَهَرَ .