بُلُوغُ الْمَرَامِ
Bulugh al-Maram 523
The Book of Prayer
وَعَنْ أَنَسٍ - رضى الله عنه - { أَنَّ اَلنَّبِيَّ - صلى الله عليه وسلم -اسْتَسْقَى فَأَشَارَ بِظَهْرِ كَفَّيْهِ إِلَى اَلسَّمَاءِ } أَخْرَجَهُ مُسْلِمٌ [1] .
Narrated Anas (RA): The Prophet (ﷺ) prayed for rain pointing the back of his palms to the sky. [Reported by Muslim].
Sunnah.com reference: Book 2, Hadith 454