It is narrated that a fire worshipper sat at one place for forty years worshipping fire. After forty years, when he rose from that place, people started using dirt from under his feet to cure eye infections.
A saint became worried when he saw this and wondered how a falsifier was bestowed such a miracle. He was uncertain about this matter.
Then he heard a voice, ‘Why are you worried; just know that God did not waste the labour of an untrue and misled individual. Imagine what will be the reward if a truthful person tries to approach Him, and how much reward will he be bestowed?
(Malfuzat, Vol. 9, p. 246–247)