Riyad as-Salihin 17:270

'Uqbah bin 'Amir (May Allah be pleased with him) said:

The Messenger of Allah (ﷺ) said, "A Mu'min is the brother of another Mu'min; and thus it is not permissible for a Mu'min to make an offer while the offer of his brother is pending, nor should he make a proposal of marriage while that of his brother is pending till he withdraws his proposal." [Muslim] .