Sahih Muslim
The Book of Transactions (Kitab Al-Buyu)
Book Number: 010 / Hadith Number: 3618
Ibn 'Umar (Allah be pleased with them) reported Allah's Messenger (may peace be upon him) as saying: A person should not enter into a transaction when his brother is already making a transaction and he should not make a proposal of marriage when his brother has already made a proposal except when lie gives permission.