Sayyiduna Anas (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) passed by the people of al-Baqi' and said: "O people of al-Baqi'!" They raised their heads (to listen), and he (peace and blessings be upon him) said: "O people of al-Baqi'! Two parties to a sale must not part without mutual consent."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10447