Sahih Al Bukhari Hadith Number 1026

Narrated `Abbad bin Tamim from his uncle who said: “The Prophet (peace be upon him) invoked Allah for rain and offered a two rak`at prayer and he put his cloak inside out.”

15  Invoking Allah for Rain (Istisqaa)
Hadith number 1026
Volume no 2
Chapter: 18 (The Istisqa’ prayer consists of two Rak’a)