Compiled by Imam Yahya ibn Sharaf al-Nawawi
وعن ابن عمر رضي الله عنهما، أن رسول الله صلى الله عليه وسلم قال: "لا يبع بعضكم على بيع بعض، ولا يخطب على خطبة أخيه إلا أن يأذن له" ((متفق عليه، وهذا لفظ مسلم))
The Messenger of Allah (ﷺ) said, "A person should not enter into a transaction when his (Muslim) brother has already negotiated, nor should he make a proposal of marriage when that of his brother is pending, except with the permission of the latter."[Al-Bukhari and Muslim]