Riyad as-Salihin 17:269

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]