"None of you should propose marriage to a woman when someone else has already proposed to her, unless he marries (and he gives up the idea), or gives him permission."  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ لاَ يَخْطُبْ أَحَدُكُمْ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَنْكِحَ أَوْ يَتْرُكَ  

[Machine] "The Messenger of Allah ﷺ forbade accepting gifts, selling goods for others, and no one should propose to a woman who is already engaged until the engaged person gets married or breaks off the engagement."  

«نَهَى رَسُولُ اللهِ ﷺ أَنْ يُتَلَقَّى الْأَجْلَابُ وَلَا يَبِيعَ حَاضِرٌ لِبَادٍ وَلَا يَخْطُبَ أَحَدُكُمْ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَنْكِحَ أَوْ يَدَعَ»  

"لَا يَخْطُب أَحَدُكُمْ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَنْكِحَ أَوْ يَتْرُكَ".