One amongst you should not enter into a transaction when another is bargaining.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ لاَ يَبِعْ بَعْضُكُمْ عَلَى بَيْعِ بَعْضٍ  

A person should not enter into a transaction when his brother is already making a transaction and he should not make a proposal of marriage when his brother has already made a proposal except when he gives permission.  

عَنِ النَّبِيِّ ﷺ قَالَ لاَ يَبِعِ الرَّجُلُ عَلَى بَيْعِ أَخِيهِ وَلاَ يَخْطُبْ عَلَى خِطْبَةِ أَخِيهِ إِلاَّ أَنْ يَأْذَنَ لَهُ  

A Muslim should not purchase (in opposition) to his brother.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ لاَ يَسُمِ الْمُسْلِمُ عَلَى سَوْمِ أَخِيهِ  

This hadith has been narrated on the authority of Abu Huraira through another chain of transmitters but with a slight change of words.  

عَنِ النَّبِيِّ ﷺ ح وَحَدَّثَنَاهُ مُحَمَّدُ بْنُ الْمُثَنَّى حَدَّثَنَا عَبْدُ الصَّمَدِ حَدَّثَنَا شُعْبَةُ عَنِ الأَعْمَشِ عَنْ أَبِي صَالِحٍ عَنْ أَبِي هُرَيْرَةَ عَنِ النَّبِيِّ ﷺ ح وَحَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ حَدَّثَنَا أَبِي حَدَّثَنَا شُعْبَةُ عَنْ عَدِيٍّ وَهُوَ ابْنُ ثَابِتٍ عَنْ أَبِي حَازِمٍ عَنْ أَبِي هُرَيْرَةَ أَنَّ رَسُولَ اللَّهِ ﷺ نَهَى أَنْ يَسْتَامَ الرَّجُلُ عَلَى سَوْمِ أَخِيهِ وَفِي رِوَايَةِ الدَّوْرَقِيِّ عَلَى سِيمَةِ أَخِيهِ  

Do not go out to meet riders to enter into transaction with them; none of you must buy in opposition to another, nor must you bid against one another; a townsman must not sell for a man from the desert, and do not tie up udders of carnels and sheep, and he who buys them after that has been done has two courses open to him: after he has milked them he may keep them if he is pleased with them, or he may return them along with a sit of dates if he is displeased with them.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ لاَ يُتَلَقَّى الرُّكْبَانُ لِبَيْعٍ وَلاَ يَبِعْ بَعْضُكُمْ عَلَى بَيْعِ بَعْضٍ وَلاَ تَنَاجَشُوا وَلاَ يَبِعْ حَاضِرٌ لِبَادٍ وَلاَ تُصَرُّوا الإِبِلَ وَالْغَنَمَ فَمَنِ ابْتَاعَهَا بَعْدَ ذَلِكَ فَهُوَ بِخَيْرِ النَّظَرَيْنِ بَعْدَ أَنْ يَحْلُبَهَا فَإِنْ رَضِيَهَا أَمْسَكَهَا وَإِنْ سَخِطَهَا رَدَّهَا وَصَاعًا مِنْ تَمْرٍ  

Abu Huraira (Allah be pleased with him) reported that Messenger of Allah ﷺ forbade the (people) meeting the caravan (for entering into business transaction with them), and the selling of goods by a townsman on behalf of a man of the desert, and seeking by a woman the divorce of her sister (from her husband), and outbidding (against one another), and tying up the udders (of animals), and buying of (things) in opposition to one's brother.  

أَنَّ رَسُولَ اللَّهِ ﷺ نَهَى عَنِ التَّلَقِّي لِلرُّكْبَانِ وَأَنْ يَبِيعَ حَاضِرٌ لِبَادٍ وَأَنْ تَسْأَلَ الْمَرْأَةُ طَلاَقَ أُخْتِهَا وَعَنِ النَّجْشِ وَالتَّصْرِيَةِ وَأَنْ يَسْتَامَ الرَّجُلُ عَلَى سَوْمِ أَخِيهِ  

This hadith has been narrated through another chain of transmitters.  

حَدَّثَنَا شُعْبَةُ بِهَذَا الإِسْنَادِ فِي حَدِيثِ غُنْدَرٍ وَوَهْبٍ نُهِيَ وَفِي حَدِيثِ عَبْدِ الصَّمَدِ أَنَّ رَسُولَ اللَّهِ ﷺ نَهَى بِمِثْلِ حَدِيثِ مُعَاذٍ عَنْ شُعْبَةَ  

Ibn Umar (Allah be pleased with them) reported that Messenger of Allah ﷺ forbade the outbidding (against another).  

أَنَّ رَسُولَ اللَّهِ ﷺ نَهَى عَنِ النَّجْشِ