Hadith: The Messenger of Allah (may Allah's peace and blessings be upon him) forbade Munābadhah, which is when a man throws a garment to another thus selling it to him before the buyer unfolds it or examines it. He also forbade Mulāmasah, which is when a man touches a garment [thus buying it] without examining it

عن أبي سعيد الخدري رضي الله عنه مرفوعاً: «أن رسول الله صلى الله عليه وسلم نهى عن الْمُنَابَذَةِ-وهي طرح الرجل ثوبه بالبيع إلى الرجل قبل أن يقلبه، أو ينظر إليه-، ونهى عن الْمُلَامَسَةِ -والملامسة: لمس الرجل الثوب ولا ينظر إليه-».
[صحيح] - [متفق عليه]

Abu Sa‘īd al-Khudri (may Allah be pleased with him) reported; The Messenger of Allah (may Allah's peace and blessings be upon him) forbade Munābadhah, which is when a man throws a garment to another thus selling it to him before the buyer unfolds it or examines it. He also forbade Mulāmasah, which is when a man touches a garment, thus buying it without examining it.
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) forbade any sale that involves Gharar (uncertainty, risk), because of the harm it may incur to one of the contracting parties through a possible injustice in its sale. This can occur when the condition of the item is unknown to the seller, the buyer, or both of them. It can occur in Munābadhah, where the seller throws a garment, for example, to the buyer thereby concluding the sale before the garment has been examined closely or turned over. Likewise in Mulāmasah, which is making a sale effective upon touching a garment, for example, before examining it closely or turning it over. These two contracts lead to uncertainty and deceit regarding the commodity being sold and bought. Either the seller or the buyer is at risk of gaining or losing, thereby engaging in “gambling”, which is prohibited.