Muwatta Malik
Purity
Hadith #75
2.75
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ إِذَا جَاوَزَ الْخِتَانُ الْخِتَانَ فَقَدْ وَجَبَ الْغُسْلُ .
Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to say, "When the circumcised part passes the circumcised part, ghusl is obligatory