Abd al-Rahman bin Shimasah al-Mahri (may Allah have mercy on him) heard Sayyiduna Uqbah bin Amir (may Allah be pleased with him) say that the Messenger of Allah (peace and blessings be upon him) said: "The believer is the brother of the believer, and it is not lawful for a believer to buy over the sale of his brother until he leaves it, nor to propose over his proposal until he leaves it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10899