Abu Salamah bin Abd al-Rahman (may Allah have mercy on him) says: I asked Sayyidah Aishah (may Allah be pleased with her): What makes ghusl obligatory? She said: O Abu Salamah, do you know what your example is? Your example is that of a chick who sees the rooster crowing and cries out along with it. "When the private part meets the private part, ghusl becomes obligatory."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 781