Sayyiduna Uqbah bin Amir (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "The believer is the brother of the believer. It is not permissible for a believer to buy over the sale of his brother until he leaves it, nor to propose marriage over his proposal until he leaves it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14944