Imam Malik's Muwatta, Chapter: Purity

Yahya related to me from Malik from Yahya ibn Said from Sulayman ibn Yasar that Umar ibn al-Khattab led the people in the subh prayer and then went out to his land in Juruf and found semen on his clothes. He said, "Since we have been eating rich meat our veins have become fulsome." He did ghusl, washed the semen from his clothing, and did his prayer again.