Muwatta Imam Malik Book of Purity, Hadith 77

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."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ إِذَا جَاوَزَ الْخِتَانُ الْخِتَانَ فَقَدْ وَجَبَ الْغُسْلُ .