If the people went out for the prayer for rain (istisqa’) or intended to
do so and it rained, then there can be two cases:
- If they were prepared to go out and it rained before they left,
then they should thank Allah (subhanahu wa ta‘ala) for His
blessings and not leave.
- If they had already come out, they should pray in thankfulness
to Allah (subhanahu wa ta‘ala).(Al-Mughni, 2/296)