Sahih Bukhari, Chapter: Invoking Allah for Rain (Istisqaa)

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.