The Prophet ﷺ said, "None of you should sell over the sale of his brother, nor propose over the proposal of his brother, unless he gives him permission."
حدثنا يحيى عن عبيد الله حدثني نافع
عن النبي ﷺ قال لا يبع أحدكم على بيع أخيه ولا يخطب على خطبة أخيه إلا أن يأذن له
Its isnad is sahih. Abu Dawud (2:189) narrated it through Ibn Numayr from Ubaydullah from Nafi, with similar wording. Munziri said, Muslim and Ibn Majah also related it. It is found in Sahih Muslim (1:399) through Yahya from Ubaydullah.The prohibition against outbidding one's brother in a sale has already come up during hadith 4531 through Malik from Nafi. The prohibition against proposing marriage over one's brother's proposal was narrated by Malik in Muwatta (2:61-62) from Nafi.