اللّهُمَّ اسْقِـنا غَيْـثاً مُغيـثاً مَريئاً مُريـعاً، نافِعـاً غَيْـرَ ضار، عاجِـلاً غَـيْرَ آجِل
Allaahumma asqinaa ghaythan mugheethan maree an maree an, naafi an ghayradhaarrin, aajilan ghayra aajilin.
O Allah, shower upon us abundant rain, beneficial not harmful, swiftly and not delayed.
Reference: Abu Dawud 1/303. See also Al-Albani Sahih Abu Dawud 1/216.