The Book of Marriage
Ṣaḥīḥ Muslim 3455
لاَ يَبِعِ الرَّجُلُ عَلَى بَيْعِ أَخِيهِ وَلاَ يَخْطُبْ عَلَى خِطْبَةِ أَخِيهِ إِلاَّ أَنْ يَأْذَنَ لَهُ
lā yabiʿi ar-rajulu ʿalā bayʿi akhīhi walā yakhṭub ʿalā khiṭbah akhīhi illā an yaʾdhana lah
Transliteration generated from the Arabic vowel marks
The full narration
Ibn Umar (Allah be pleased with them) reported Allah's Apostle (ﷺ) as having said this:A person should not enter into a transaction when his brother (had already entered into but not finalised), and he should not make proposal of marriage upon the proposal already made by his brother, until he permits it
Source
Ṣaḥīḥ Muslim · 3455
Ṣaḥīḥ