The Coalition - 33:37
"You used to say to the one to whom Allah had done good to you and you had done good to him, "Keep your wife and observe Allah". Thus you were concealing what Allah would reveal. You were afraid of the people when you should have been afraid of Allah. And when Zayd was estranged from his wife, We gave you her in marriage so that the believers may not find it difficult to marry their adopted children when they are estranged from their wives. Allah´s command must be fulfilled."
The rules applicable to adopted children are different from the rules applicable to natural children:
The Coalition/33:4
Allah has not put two hearts in any man. He has not made your wives your mothers, whom you forbade yourselves by likening them to your mothers, nor has He made your adopted children your own children. All this is but the words of your mouths. Allah speaks the truth and He guides.
The Coalition/33:37
"You used to say to the one to whom Allah had done good to you and you had done good to him, "Keep your wife and observe Allah". Thus you were concealing what Allah would reveal. You were afraid of the people when you should have been afraid of Allah. And when Zayd was estranged from his wife, We gave you her in marriage so that the believers may not find it difficult to marry their adopted children when they are estranged from their wives. Allah´s command must be fulfilled."