Sayyiduna Anas (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) bought Sayyidah Safiyyah (may Allah be pleased with her) from Dihyah al-Kalbi (may Allah be pleased with him) for seven slaves.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10523