كتاب الطهارة
2
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.

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنْ سُلَيْمَانَ بْنِ يَسَارٍ، أَنَّ عُمَرَ بْنَ الْخَطَّابِ، صَلَّى بِالنَّاسِ الصُّبْحَ ثُمَّ غَدَا إِلَى أَرْضِهِ بِالْجُرُفِ فَوَجَدَ فِي ثَوْبِهِ احْتِلاَمًا فَقَالَ إِنَّا لَمَّا أَصَبْنَا الْوَدَكَ لاَنَتِ الْعُرُوقُ ‏.‏ فَاغْتَسَلَ وَغَسَلَ الاِحْتِلاَمَ مِنْ ثَوْبِهِ وَعَادَ لِصَلاَتِهِ ‏.‏
USC-MSA web (English) reference : Book 2, Hadith 84
Arabic reference : Book 2, Hadith 114