سنن النسائي

It was narrated from Hafs bin 'Asim, from Abu Hurairah that the Prophet (ﷺ) 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.