One goes out for salaat il-istisqa’ 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:

1. 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.
2. If they had already come out, they should pray in thankfulness
   to Allah (subhanahu wa ta‘ala).(Al-Mughni, 2/296)