Yahya related to me from Malik from Ismail ibn Abi Hakim fromSulayman ibn Yasar that Umar ibn al-Khattab went out early in themorning to his land in al-Juruf and found semen on his garment. Hesaid, "I have been tried with wet dreams since I have been entrustedwith governing the people." He did ghusl and washed his garment ofwhat he saw of the semen, and then prayed after the sun had risen.
USC-MSA web (English) reference: Book 2, Hadith 83
Arabic reference: Book 2, Hadith 113