The Messenger of Allah ﷺ said, "The standard of weight is the weight used by the people of Mecca, and the standard of measure is the measure used by the people of Medina."
قال رسول الله ﷺ: الوزن وزن أهل مكة، والمكيال مكيال أهل المدينة.
Abu Dawud (3340), Nasai (2521, 4594), Bayhaqi (6/31), and Abd bin Humayd (803) all narrated it through the route of Abu Naeem Fadl bin Dukayn Malai, who said, our teacher Sufyan narrated to us, from Hanzala bin Abu Sufyan, from Tawus, from Ibn Umar, and he mentioned it. Its chain of transmission is sahih.Abu Dawud said, "Firyabi and Abu Ahmad likewise narrated it from Sufyan, agreeing with them both on the wording." He said, "Abu Ahmad narrated it with Ibn Abbas in place of Ibn Umar. Walid bin Muslim narrated it from Hanzala, saying, 'The weight is that of Madinah, and the measure is that of Makkah.'" He also said, "There is disagreement over the wording in the hadith of Malik bin Dinar, from Ataa, from the Prophet ﷺ, concerning this," meaning as a mursal narration.What Daraqutni mentioned in Al-Ilal (13/126) is that Abu Ahmad Zubayri diverged only in the chain of transmission, saying, "from Ibn Abbas" instead of Ibn Umar. Daraqutni said, "The sahih version is from Ibn Umar." Firyabi diverged in the wording, saying, "The measure is the measure of the people of Makkah, and the weight is the weight of the people of Madinah." Daraqutni said, "The sahih version is what has already been mentioned."As for Bayhaqi, he narrated it through the route of Abu Ahmad Zubayri, saying, "from Ibn Abbas, from the Prophet ﷺ, who said, 'The measure is the measure of the people of Makkah, and the weight is the weight of the people of Madinah.'" Bayhaqi said, "This diverges from Abu Naeem in the wording of the hadith, and the correct version is what Abu Naeem narrated, both in chain and wording." And Allah knows best what is correct.The meaning of the hadith, in brief, is that when extracting zakat on gold and silver, one looks to the scale of the people of Makkah, which was standardized in the time of Abdul-Malik bin Marwan when he wished to mint dinars and dirhams. The minimum threshold requiring zakat was two hundred dirhams, and on this basis all dirhams are measured across different lands, even though their weights vary from one land to another.As for matters relating to the obligation of atonements, the payment of Zakat al-Fitr, the calculation of maintenance payments, and similar matters, one looks to the measure of the people of Madinah, which is what is called the saa. The saa of the people of Madinah differs from the saa of the people of Iraq, for the saa of the people of Madinah is five and a third pounds by the Iraqi standard, while the saa of the people of Iraq is eight pounds.As for transactions, the saa customarily recognized among the people of one's own land is what applies, but when the matter pertains to the Shariah and its rulings, it is the saa of the people of Madinah.