Yahya related to me from Malik from Hisham ibn Urwa from hisfather fromYahyaibn Abd ar-Rahman ibn Ha ib that he had set off forMumra with Umar ibn al-Khattab in a party of riders, among whom wasAmr ibn al-As. Umar ibn al-Khattab dismounted for a rest late at nighton a certain road near a certain oasis. Umar had a wet dream when itwas almost dawn and there was no water among the riding party. He rodeuntil he came to some water and then he began to wash off what he sawof the semen until it had gone. Amr ibn al-As said to him, "It ismorning and there are clothes with us, so allow your garment to bewashed. ''Umar ibn al-Khattab said to him, "I am surprised at you, Amribn al-As! Even if you could find clothes, would everybody be able tofind them? By Allah, if I were to do it, it would become a sunna. No,I wash what I see, and I sprinkle with water what I do not see."Malik spoke about a man who found traces of a wet dream on hisclothes and did not know when it had occurred and did not rememberanything he had seen in his sleep. He said, "Let the intention of hisghusl be from the time when he last slept, and if he has prayed sincethat last sleep he should repeat it. This is because often a man has awet dream and sees nothing, and often he sees something but does nothave an emission. But, if he finds liquid on his garment he must doghusl. This is because Umar repeated what he had prayed after the timehe had last slept and not what was before it."
USC-MSA web (English) reference: Book 2, Hadith 85
Arabic reference: Book 2, Hadith 115