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]
App reference: Book 18, Hadith 269
Arabic/English book reference: Book 18, Hadith 1779