It is narrated from Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) forbade the selling and gifting of wala'.
Hadith Referenceمسند الإمام الشافعي / كتاب العتق والولاء والمدبر والمكاتب وحسن الملكة / 1089