Hadith: The Messenger of Allah (may Allah's peace and blessings be upon him) prohibited the sale of exchange of gold with silver on credit

عن أبي المنهال قال: «سألت البراء بن عازب، وزيد بن أرقم، عن الصَّرْفِ؟ فكل واحد يقول: هذا خير مني. وكلاهما يقول: نهى رسول الله صلى الله عليه وسلم عن بيع الذهب بالوَرِقِ دَيْنَاً».
[صحيح] - [متفق عليه]

Abu al-Minhāl said: “I asked Al-Barā’ ibn ‘Āzib and Zayd ibn Arqam about ‘Sarf’ (exchanging monies). Each of them said about the other: 'He is better than me’, and both of them said: ‘The Messenger of Allah (may Allah's peace and blessings be upon him) prohibited the sale of exchange of gold with silver on credit.'"
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

Abu al-Minhāl asked al-Barā’ ibn ‘Āzib and Zaid ibn Arqam (may Allah be pleased with him) about 'Sarf', which means exchanging monies or, in other words, selling the value of one for the other. At first each of them out of piety and humility referred the question to the other as being more knowledgeable and more eligible to answer it. When they answered with both of them giving the same answer which they heard from the Prophet (may Allah's peace and blessings be upon him) that he prohibited selling gold for silver on credit because they are both prone to usury. When this is the case, then it is necessary that they are exchanged on the spot in the same session before the two parties depart; otherwise, the exchange is invalid and it is considered Nasī’ah usury (usury related to deferment).