Sahih Al Bukhari Hadith Number 1019

Narrated Anas bin Malik: A man came to Allah’s Messenger (peace be upon him) and said, “O Allah’s Messenger (peace be upon him)! Livestock are destroyed and the roads are cut off; so please invoke Allah.” So Allah’s Messenger (peace be upon him) prayed for rain and it rained from that Friday till the next Friday. Then a man came to the Prophet (peace be upon him) and said, “O Allah’s Messenger (peace be upon him)! The houses have collapsed, roads are cut off and the livestock are destroyed.” So Allah’s Messenger (peace be upon him) said, “O Allah ! (Let it rain) on the tops of the mountains, on the plateaus, in the valleys and over the places where trees grow.” So the clouds cleared away from Medina as clothes are taken off.

15  Invoking Allah for Rain (Istisqaa)
Hadith number 1019
Volume no 2
Chapter: 12 (If the people request the Imam to invoke Allah for rain, the Imam should not refuse)