Reference : Hisn al-Muslim 169

اللّهُمَّ اسْقِـنا غَيْـثاً مُغيـثاً مَريئاً مُريـعاً،
نافِعـاً غَيْـرَ ضار،
عاجِـلاً غَـيْرَ آجِل
Allāhumma ‘asqinā ghaythan mughīthan marī'an murī`a, 
nāfi`an ghayra ḍārr, 
`ājilan ghayra ājil.

O Allah, shower upon us abundant rain, beneficial not harmful, swiftly and not delayed.

Reference:
Abu Dawud 1/303. See also Al-Albani SahihAbu Dawud 1/216.

Chapter: For rain