It is narrated from Sa'id bin Al-Musayyib (may Allah have mercy on him) that Abu Musa Ash'ari (may Allah be pleased with him) came to Umm al-Mu'mineen Aisha (may Allah be pleased with her) and said: "The difference of opinion among the companions of Muhammad ﷺ on an issue has become burdensome for me, and I consider it important to present it before you." Aisha (may Allah be pleased with her) said: "What is that issue? Why do you not want to ask your mother about it? Ask me!" So Abu Musa Ash'ari (may Allah be pleased with him) said to Aisha (may Allah be pleased with her): "A man goes to his wife and withdraws before ejaculation." Aisha (may Allah be pleased with her) said: "When the circumcised part passes the circumcised part, ghusl becomes obligatory." Abu Musa (may Allah be pleased with him) said: "After you, I will never ask anyone (about this issue) ever again."
Hadith Referenceمسند الإمام الشافعي / كتاب الطهارة / 95
Hadith Takhrijصحيح: اخرجه البيهقي في المعرفة السنن والآثار (250) وعبد الرزاق (954)