Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) "forbade the sale of the offspring of the pregnant one (habal al-habalah)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10861