Sahih Bukhari Volume 002, Book 017, Hadith Number 123.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 17. Invoking Allah for Rain (Istisqaa)
Narated By 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.