Musnad Ahmad 32:569

Mu'ammal ibn Isma'il narrated to us, Hammad ibn Zayd narrated to us, Thabit narrated to us, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) came to the house of Zayd ibn Harithah and saw his wife, Zaynab, and it was as if something entered him—I do not know if this is from the statement of Hammad or if it is in the hadith. Zayd came complaining about her to him, so the Prophet (peace and blessings of Allah be upon him) said to him: "Keep your wife to yourself, and fear Allah." He said: Then it was revealed: "And you fear Allah, while you conceal within yourself that which Allah is to disclose" [Al-Ahzab: 37], up to His saying: "We gave her to you in marriage" [Al-Ahzab: 37], meaning Zaynab.