(3) باب سُؤَالِ النَّاسِ الإِمَامَ الاِسْتِسْقَاءَ إِذَا قَحَطُوا
Narrated Anas:
Whenever drought threatened them, `Umar bin Al-Khattab, used to ask Al-Abbas bin `Abdul Muttalib to invoke Allah for rain. He used to say, "O Allah! We used to ask our Prophet to invoke You for rain, and You would bless us with rain, and now we ask his uncle to invoke You for rain. O Allah ! Bless us with rain."(1) And so it would rain.
Reference: Sahih al-Bukhari 1010
In-book reference: Book 15, Hadith 5
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 17, Hadith 123