Imam Malik's Muwatta, Chapter: Purity

Yahya related to me from Malik from Ismail ibn Abi Hakim from Sulayman ibn Yasar that Umar ibn al-Khattab went out early in the morning to his land in al-Juruf and found semen on his garment. He said, "I have been tried with wet dreams since I have been entrusted with governing the people." He did ghusl and washed his garment of what he saw of the semen, and then prayed after the sun had risen.