It happened that soon after the official nomination of Imam Reza (A.S.) as the caliph's successor, there was no rain for quite some time. The enemies seized this natural phenomenon as an opportunity to portray it as an ill omen resulting from Mamuns' declaration. Mamun, on hearing the rumor became upset, and requested the Holy Imam(A.S.) to pray to Almighty Allah to deliver the people from the drought. The Holy Imam(A.S.) obliged, and on Monday morning with a large number of people, he headed towards the outskirts of the capital city Marw to pray.
Praising Almighty Allah, the Holy Imam(A.S.) started praying for rain. Soon some clouds appeared accompanied by thunder and lightning much to the joy of the people. But the Holy Imam(A.S.) asked them to be patient saying that these clouds were not for them. The clouds passed and were followed by more clouds, also accompanied with thunder and lightning. Imam Reza (A.S.) said that these clouds again were not for Marw but for such and such a place. More clouds followed, and on the eleventh occasion, the Holy Imam(A.S.) said that this time it was for Marw and by the will of Almighty Allah, it would start raining by the time the people reach their houses.
After the people reached their homes, by the will of Allah and the blessed prayers of Imam Reza (A.S.), the downpour started and it rained so heavily that the fountains, ponds, and the deserts were filled with water.
source : http://www.imamreza.net
