Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates from Sayyiduna Umar bin al-Khattab (may Allah be pleased with him): "Whoever sells a slave who has property, the slave's property belongs to the seller, unless the buyer stipulates (otherwise)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10761