Reference : Hisn al-Muslim 170

اللّهُمَّ أغِثْنـا، 
اللّهُمَّ أغِثْنـا، 
اللّهُمَّ أغِثْنـا
Allāhumma aghithnā, 
Allāhumma aghithnā, 
Allāhumma aghithnā.


O Allah, send us rain. 
O Allah, send us rain. 
O Allah, send us rain.

Reference:
Al-Bukhari 1/224, Muslim 2/613.

Chapter: For rain