Riyad as-Salihin 1779
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 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)‏‏)‏