Calculation of Nisaab of Silver
Q: In the following article on your website: http://alhaadi.org.za/articles/qurbani/668-who-is-required-to-perform-q… is mentioned that "Every adult Muslim, male or female, who owns 613.35 grams of silver or its equivalent in money, personal ornaments, stock-in-trade or any other form of wealth which is surplus to his basic needs, is under an obligation to offer a Qurbani."
Can you please give the reference of Hadith or other authentic books as to how the figure of 613.35 has been deduced?