Qatadah (may Allah have mercy on him) said: The Prophet (peace and blessings be upon him) proposed to Zaynab (may Allah be pleased with her), who was the daughter of his paternal aunt, and he (peace and blessings be upon him) wanted her for Zayd. She thought that he (peace and blessings be upon him) wanted her for himself, and when she learned that he (peace and blessings be upon him) wanted her for Zayd, she refused. So Allah revealed: "It is not for a believing man or a believing woman, when Allah and His Messenger have decided a matter, that they should have any choice in their affair" [al-Ahzab: 36]. Then she was pleased and submitted.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20495