Ibn 'Umar (May Allah be pleased with them) said:
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].

وعن ابن عمر رضي الله عنهما، أن رسول الله ﷺ قال: "لا يبع بعضكم على بيع بعض، ولا يخطب على خطبة أخيه إلا أن يأذن له" ((متفق عليه، وهذا لفظ مسلم))