It is narrated from Muti' bin al-Aswad (may Allah be pleased with him) that Umar bin al-Khattab (may Allah be pleased with him) led the morning (Fajr) prayer; then Umar (may Allah be pleased with him) and I rode off towards our land. When Umar (may Allah be pleased with him) sat by a stream to perform ablution (wudu) from it, there was semen on his thigh. He began saying: This semen is on my thigh and I was not even aware of it. He wiped it off. Then (he said): By Allah! Since I have been eating rich, fatty food and have reached such a stage of age, something comes out of me and I am not even aware of it. Then he took a ritual bath (ghusl) and repeated the morning prayer, but he did not order a single person to repeat the prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4072