It is narrated from Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace be upon him) said: "None of you should send a marriage proposal over the proposal of his brother."
Hadith Referenceمسند الإمام الشافعي / كتاب النكاح / 1125
Hadith Takhrijاخرجه البخاري، النكاح ، باب لا يخطب على خطبة اخيه حتى ينكح او يدع (5142)، (2139) ومسلم، النكاح، باب تحريم الخطبة على خطبة اخيه حتى يأذن أو يترك (1412)۔
It is narrated from Abu Huraira (may Allah be pleased with him) from the Prophet Muhammad (peace and blessings be upon him) similar to the previous hadith. And some hadith scholars have added these words: "until he seeks permission from him or he breaks off the engagement."
Hadith Referenceمسند الإمام الشافعي / كتاب النكاح / 1126
Hadith Takhrijاخرجه البخاري، النكاح، باب لا يخطب على خطبة اخيه حتى ينكح او يدع (5144)، (2140) - ومسلم، النكاح، باب تحريم الخطبة على خطبة اخيه ..... الخ (1413)۔
It is narrated from Ibn Umar (may Allah be pleased with them) that the Messenger of Allah (peace and blessings be upon him) said: "None of you should propose to a woman to whom his brother has already proposed."
Hadith Referenceمسند الإمام الشافعي / كتاب النكاح / 1127
It is narrated from Abu Huraira (may Allah be pleased with him) that he stated: The Messenger of Allah (peace and blessings be upon him) said: "None of you should propose marriage over the proposal of his brother."
Hadith Referenceمسند الإمام الشافعي / كتاب النكاح / 1128
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)۔