Sa'id bin Wahb (may Allah have mercy on him) says: A man came to Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) and said: "My mother has made her slave-woman lawful for me." Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: "She is not lawful for you except in one of three ways: 1. an outright gift, 2. a sale, 3. marriage."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13852