Muwatta Malik 85

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ أَبِيهِ، أَنَّ عُمَرَ بْنَ الْخَطَّابِ، قَالَ إِنِّي لأَجِدُهُ يَنْحَدِرُ مِنِّي مِثْلَ الْخُرَيْزَةِ فَإِذَا وَجَدَ ذَلِكَ أَحَدُكُمْ فَلْيَغْسِلْ ذَكَرَهُ وَلْيَتَوَضَّأْ وُضُوءَهُ لِلصَّلاَةِ يَعْنِي الْمَذْىَ ‏.‏

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

Reference

Muwatta Malik 85

In-book

Book 2, Hadith 54