It is narrated from Ibn Umar (may Allah be pleased with them both) that the Prophet (ﷺ) forbade a man from proposing to a woman to whom his brother has already proposed, until he marries her or leaves her (i.e., breaks off the engagement).
Hadith Referenceمسند الإمام الشافعي / كتاب النكاح / 1129
Hadith Takhrijصحيح اخرجه البيهقى فى المعرفة السنن والآثار (4187) - واحمد: (2/ 42)۔