What should one do if people go out to congregate for
salaat il-istisqa’ (prayer asking for rain) and it rains?
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)