Maliks Muwatta Book 2, Hadith Number 83.

Section : The Repetition of the Prayer by a Person in a State of Major Ritual Impurity, his Doing Ghusl, when He has Prayed without Remembering it, and his Washing his Garments.

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.