ما هو الاحتلام؟

No, it is not considered a sin. Rather, ihtilam (wet dream) is when a sleeping person dreams that he is engaging in sexual intercourse. If he wakes up and sees water (semen) on his clothes, then ghusl (ritual bath) becomes obligatory for him. However, if he sees nothing, then nothing is incumbent upon him, based on the saying of the Prophet, peace and blessings be upon him: "Yes, if she sees water." There is no sin upon the one who experiences a wet dream, nor does it invalidate his fast, because it occurs without his will.