Reference : Hisn al-Muslim 171

اللّهُمَّ اسْقِ عِبادَكَ وَبَهـائِمَك، 
وَانْشُـرْ رَحْمَـتَكَ 
وَأَحْيِي بَلَـدَكَ المَيِّـت
Allāhumma’sqi `ibādaka wa bahā'imak, 
wanshur raḥmatak, 
wa aḥyi baladaka ‘l-mayyit


O Allah, give water to Your slaves, and Your livestock, 
and spread Your mercy, 
and revive Your dead land.

Reference:
Abu Dawud 1/305. Al-Albani graded it good in Sahih Abu Dawud 1/218.

Chapter: For rain