Maliks Muwatta Book 2, Hadith Number 85.
Yahya related to me from Malik from Hisham ibn Urwa from his father from Yahyaibn Abd ar-Rahman ibn Haib that he had set off for Mumra with Umar ibn al-Khattab in a party of riders, among whom was Amr ibn al-As. Umar ibn al-Khattab dismounted for a rest late at night on a certain road near a certain oasis. Umar had a wet dream when it was almost dawn and there was no water among the riding party. He rode until he came to some water and then he began to wash off what he saw of the semen until it had gone. Amr ibn al-As said to him, “It is morning and there are clothes with us, so allow your garment to be washed. ”Umar ibn al-Khattab said to him, “I am surprised at you, Amr ibn al-As! Even if you could find clothes, would everybody be able to find 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 his clothes and did not know when it had occurred and did not remember anything he had seen in his sleep. He said, “Let the intention of his ghusl be from the time when he last slept, and if he has prayed since that last sleep he should repeat it. This is because often a man has a wet dream and sees nothing, and often he sees something but does not have an emission. But, if he finds liquid on his garment he must do ghusl. This is because Umar repeated what he had prayed after the time he had last slept and not what was before it.”