Narrated 'Aishah [may Allah be pleased with her]:
"If the Messenger of Allah (ﷺ) was to have concealed anything that was revealed to him, then he would have concealed these Ayat: 'When you said to him on whom Allah has bestowed grace (meaning by Islam); and you have done a favor (meaning that he was a slave and you freed him) "Keep your wife to yourself, and have Taqwa of Allah." But you did hide in yourself that which Allah will make manifest, you did fear the people whereas Allah had better right that you should fear Him' up to His saying: 'And Allah's command must be fulfilled (33:37).' They said: 'He married his wife's son, so Allah revealed: 'Muhammad is not the father of any of your men, but he is the Messenger of Allah and the Last of the Prophets (33:40).' The Messenger of Allah (ﷺ) had taken (adopted) him as a son when he was small, and he remained being called 'Zaid bin Muhammad' until he grew up to adulthood, then Allah revealed: 'Call them by their fathers, then your brothers in religion and your Mawali (33:5). (Say) So-and-so, the Mawla of so-and-so, and; So-and-so, the brother of so-and-so. 'That is more just with Allah' meaning that doing that is more just to Allah."
English reference: Vol. 5, Book 44, Hadith 3207
Arabic reference: Book 47, Hadith 3512