Muwatta Malik #112

Yahya related to me from Malik from Hisham ibn Urwa that Zuyayd ibn as-Salt said, "I went with Umar ibn al-Khattab to Juruf and he looked down and noticed that he had had a wet dream and had prayed without doing ghusl. He exclaimed, 'By Allah I realise that I have had a wet dream and did not know it and have not done ghusl.' So he did ghusl and washed off whatever he saw on his garment, and sprinkled with water whatever he did not see.Then he gave the adhan or the iqama and prayed in the midmorning."