What is the ruling on someone who issues a fatwa concerning an issue, believing it to be correct, then it becomes clear to him that it was mistaken, and is he required to inform the one to whom he issued the fatwa of that?

Speaking about the religion of Allah without knowledge is one of the gravest forbidden acts. The sin of one who gives a fatwa (religious legal opinion) without knowledge falls upon the one to whom he gave the fatwa, and he must clarify his error to anyone who received it from him. Songs accompanied by music, or indecent words, or intermingling between men and women are forbidden. As for saying: "There is no god but Allah, alone, without partner; to Him belongs all sovereignty, and to Him belongs all praise, and He is over all things competent" ten times, its virtue is immense. It is equivalent to freeing a slave, or four slaves, or ten slaves. By it, good deeds are recorded, bad deeds are erased, ranks are elevated, and its reciter is protected.