One goes out for salaat il-istisqa’ and it rains
Question

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