Sayyiduna Uqbah bin Amir (may Allah be pleased with him) was saying on the pulpit that the Messenger of Allah (peace and blessings be upon him) said: "The believer is the brother of the believer, so it is not lawful for a believer to make a sale over the sale of his brother until he leaves it, nor to propose marriage over the proposal of his brother until he leaves it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14036