Yahya related to me from Zayd ibn Aslam from his father that Umar ibn al-Khattab said, "I find it dropping from me like small beads. When you find that, wash your penis and do wudu as for prayer."  

قَالَ إِنِّي لأَجِدُهُ يَنْحَدِرُ مِنِّي مِثْلَ الْخُرَيْزَةِ فَإِذَا وَجَدَ ذَلِكَ أَحَدُكُمْ فَلْيَغْسِلْ ذَكَرَهُ وَلْيَتَوَضَّأْ وُضُوءَهُ لِلصَّلاَةِ يَعْنِي الْمَذْىَ  

[Machine] I find that it descends from me like a piglet. So, if any of you find that, let them wash their private parts and perform ablution for prayer, meaning the semen.  

إِنِّي لَأَجِدُهُ يَنْحَدِرُ مِنِّي مِثْلَ الْخَزِيرَةِ فَإِذَا وَجَدَ أَحَدُكُمْ ذَلِكَ فَلْيَغْسِلْ ذَكَرَهُ وَلْيَتَوَضَّأْ وُضُوءَهُ لِلصَّلَاةِ يَعْنِي الْمَذْيَ