Sahih Bukhari Volume 005, Book 057, Hadith Number 059.
Narated By Anas : Whenever there was drought, 'Umar bin Al-Khattab used to ask Allah for rain through Al'Abbas bin 'Abdul Muttalib, saying, "O Allah! We used to request our Prophet to ask You for rain, and You would give us. Now we request the uncle of our Prophet to ask You for rain, so give us rain." And they would be given rain."