Hadith: The Messenger of Allah (may Allah's peace and blessings be upon him) forbade the sale of silver for silver and gold for gold except for equal quantities, and commanded us to buy silver with gold as we desire, and to buy gold with silver as we desire

عن أبي بكرة رضي الله عنه قال: «نهى رسول الله صلى الله عليه وسلم عن الفضة بالفضة، والذهب بالذهب، إلا سَوَاءً بسوَاءٍ، وأمرنا أن نشتري الفضة بالذهب، كيف شئنا. ونشتري الذهب بالفضة كيف شئنا، قال: فسأله رجل فقال: يدا بيد؟ فقال: هكذا سمعت».
[صحيح] - [متفق عليه]

Abu Bakrah (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) forbade the sale of silver for silver and gold for gold except for equal quantities, and commanded us to buy silver with gold as we desire, and to buy gold with silver as we desire. A man, then, asked him: "Is it (payment) to be made on the spot?" He replied: "That is what I heard (from the Messenger of Allah)."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

Since the sale of gold for gold and silver for silver is considered usury in case they are not equivalent, it has been prohibited as long as they are not of the same weight. As for selling gold for silver or silver for gold, it is permissible even if they are of a different weight. However, the validity of the transaction requires immediate exchange during the conclusion of the sale contract; otherwise, it will be considered usury for delay, which is prohibited. This is because when the items differ in their type, the difference in weight becomes permissible on condition that exchange of the items must be immediate. This ruling applies to the items sharing a common cause of usury.