Muwatta Malik 86

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ جُنْدَبٍ، مَوْلَى عَبْدِ اللَّهِ بْنِ عَيَّاشٍ أَنَّهُ قَالَ سَأَلْتُ عَبْدَ اللَّهِ بْنَ عُمَرَ عَنِ الْمَذْىِ، فَقَالَ إِذَا وَجَدْتَهُ فَاغْسِلْ فَرْجَكَ وَتَوَضَّأْ وُضُوءَكَ لِلصَّلاَةِ ‏.‏

Yahya related to me from Malik from Zayd ibn Aslam from his father that Jundub, the mawla of Abdullah ibn Ayyash, said, "I asked Abdullah ibn Umar about prostatic fluid and he said, 'When you find it, wash your genitals and do wudu as for prayer

Reference

Muwatta Malik 86

In-book

Book 2, Hadith 55