The Messenger of Allah ﷺ said, "A man must not propose marriage over the proposal of his brother."
حدثنا أحمد بن عمرو بن السرح حدثنا سفيان عن الزهري عن سعيد بن المسيب
قال رسول الله ﷺ لا يخطب الرجل على خطبة أخيه
Bukhari recorded it in the Book of Sales, Chapter "One should not undersell his brother, nor propose in the marriage negotiation of his brother, until he permits him or gives it up," hadith no. 2140, and Muslim recorded it in the Book of Marriage, Chapter "The prohibition of proposing to a woman who has already been proposed to by another until he permits him or gives it up," hadith no. 1413, both from the hadith of Sufyan bin Uyayna, with this chain.