اللَّهُمَّ اسْقِنَا غَيْثًا مُغِيثًا ، مَرِيئًا مُرِيعًا ، نَافِعًا غَيْرَ ضَارٍّ ، عَاجِلًا غَيْرَ آجِلٍ
Translation 
O Allah, send upon us helpful, wholesome and healthy rain, beneficial not harmful rain, now, not later.
Transliteration
allaahummas-qinaa ghaythan mugheethan, maree’an muree‛an, naafi‛an ghayra ḍaarin, ‛aajilan ghayra aajil
Sources:
Abu Dawud No# 1169
