Imam Malik's Muwatta, Chapter: Purity

Yahya related to me from Malik from Yahya ibn Said from Abdullah ibn Kab, the mawla of Uthman ibn Affan that Mahmud ibn Labid al-Ansari asked Zayd ibn Thabit about a man who penetrated his wife but became listless and did not ejaculate. Zayd ibn Thabit said, "He does ghusl." Mahmud said to him, "Ubayy ibn Kab used not to think that ghusl was necessary," but Zayd ibn Thabit said, "Ubayy ibn Kab drew away from that before he died."