Sayyidah Umm Hani (may Allah be pleased with her) said: The Prophet (peace and blessings be upon him) sent me a proposal of marriage. I offered him my excuse and he accepted my excuse. Then Allah Almighty revealed the verse: "O Prophet, indeed We have made lawful for you your wives." [Surah al-Ahzab: 50]
She said: So I was not lawful for him, because I had not emigrated with him; I was one of the Tulaqa' (those set free at the conquest of Makkah).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13350