Muhammad taking his son's wife

And when you said to the one whom Allah had blessed and whom you had favoured, “Keep your wife and be mindful of Allah,” while concealing within yourself what Allah was to reveal, fearing people, though Allah is more deserving of your fear. When Zayd had ended his marriage, We married her to you so that believers would have no hesitation regarding marriage to the former wives of their adopted sons. Allah’s command is always fulfilled.

33:37And when thou saidst unto him on whom Allah hath conferred favour and thou hast conferred favour: Keep thy wife to thyself, and fear Allah. And thou didst hide in thy mind that which Allah was to bring to light, and thou didst fear mankind whereas Allah hath a better right that thou shouldst fear Him. So when Zeyd had performed that necessary formality (of divorce) from her, We gave her unto thee in marriage, so that (henceforth) there may be no sin for believers in respect of wives of their adopted sons, when the latter have performed the necessary formality (of release) from them. The commandment of Allah must be fulfilled.

Allah has not placed two hearts within any person, nor has He made your wives, whom you equate with your mothers, truly your mothers. Likewise, He has not made your adopted sons your actual sons. These are mere words from your mouths, but Allah speaks the truth and guides to the right path.

33:4Allah hath not assigned unto any man two hearts within his body, nor hath He made your wives whom ye declare (to be your mothers) your mothers, nor hath He made those whom ye claim (to be your sons) your sons. This is but a saying of your mouths. But Allah saith the truth and He showeth the way.

Call them by their fathers’ names, that is more just in the sight of Allah. But if you do not know their fathers, then they are your brothers in faith and your companions. There is no blame upon you for unintentional mistakes, but what your hearts deliberately intend.

33:5Proclaim their real parentage. That will be more equitable in the sight of Allah. And if ye know not their fathers, then (they are) your brethren in the faith, and your clients. And there is no sin for you in the mistakes that ye make unintentionally, but what your hearts purpose (that will be a sin for you). Allah is ever Forgiving, Merciful.