It is stated in the letter of Hazrat Mu'adh (may Allah be pleased with him): "Any people who enslave someone while they were previously free and weak neighbors—so whoever is confined in someone's house and brought into Islam in this way, he is a slave; and whoever has continued to pay tribute while being free, he is free. And any slave who comes running towards Muslim women in Islam becomes free."
Hadith Referenceسنن سعید بن منصور / كتاب الجهاد / 3771