مُوَطَّأُ مَالِكٍ
Muwatta Malik 86
Purity
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."
Arabic/English book reference: Book 2, Hadith 56