It is narrated from Mujahid that Ali (may Allah be pleased with him) said: "Wala' is like a contract (agreement); I will maintain it exactly where Allah the Exalted has placed it."
Hadith Referenceمسند الإمام الشافعي / كتاب العتق والولاء والمدبر والمكاتب وحسن الملكة / 1087
Hadith Takhrijاسناده صحيح : اخرجه البيهقي : 10 / 294 - وفى المعرفة السنن والآثار له ، رقم: 6054 - وعبدالرزاق (16140) وابن ابي شيبة (31600)۔
It is narrated from Abdullah bin Umar (may Allah be pleased with them) that the Messenger of Allah (peace be upon him) forbade selling or gifting of wala'.
Hadith Referenceمسند الإمام الشافعي / كتاب العتق والولاء والمدبر والمكاتب وحسن الملكة / 1088
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
It is narrated from Ibn Umar (may Allah be pleased with them both) that the Prophet (ﷺ) said: "Wala' is a bond just like the bond of lineage; it should neither be sold nor gifted."
Hadith Referenceمسند الإمام الشافعي / كتاب العتق والولاء والمدبر والمكاتب وحسن الملكة / 1090
Hadith Takhrijاخرجه البيهقي: 10/ 292 - وفى المعرفة السنن والآثار له (6053) - وصححه الحاكم: 4/ 341 - وابن حبان۔