It was narrated from Hafs bin 'Asim, from Abu Hurairah that the Prophet forbade two kids of transactions:Munabadhah and Mulamasha. And he said that Mulamasah means when one man says to another: "I will sell you my garment for your garment," and neither of them looks at the garment of the others, rather he just touches it. And Munabadhah is when he says: "I will throw what I have and you throw what you have," so that they buy from one another, and neither of them knows how much the other has, and so on.
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا الْمُعْتَمِرُ، قَالَ سَمِعْتُ عُبَيْدَ اللَّهِ، عَنْ خُبَيْبٍ، عَنْ حَفْصِ بْنِ عَاصِمٍ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ نَهَى عَنْ بَيْعَتَيْنِ أَمَّا الْبَيْعَتَانِ فَالْمُنَابَذَةُ وَالْمُلاَمَسَةُ وَزَعَمَ أَنَّ الْمُلاَمَسَةَ أَنْ يَقُولَ الرَّجُلُ لِلرَّجُلِ أَبِيعُكَ ثَوْبِي بِثَوْبِكَ وَلاَ يَنْظُرَ وَاحِدٌ مِنْهُمَا إِلَى ثَوْبِ الآخَرِ وَلَكِنْ يَلْمِسُهُ لَمْسًا وَأَمَّا الْمُنَابَذَةُ أَنْ يَقُولَ أَنْبُذُ مَا مَعِي وَتَنْبُذُ مَا مَعَكَ لِيَشْتَرِيَ أَحَدُهُمَا مِنَ الآخَرِ وَلاَ يَدْرِي كُلُّ وَاحِدٍ مِنْهُمَا كَمْ مَعَ الآخَرِ وَنَحْوًا مِنْ هَذَا الْوَصْفِ .
Nesai
The Book of Financial Transactions • Hadith 4517