Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.014.

There is no zakah on silver until the amount exceeds two hundred dirhams. The amount payable is a quarter of a tenth for any amount. There is no zakah exemption on (silver) coins if they attain a nisab.

‘Ali reported that the Prophet, upon whom be peace, said: “I exempt you from paying zakah on horses and slaves. Pay, then, zakah on silver, one dirham for each forty dirhams. Zakah is not due on ninety or one hundred dirhams of silver. If it reaches two hundred dirhams, five dirhams are to be paid.” This was related by the authors of as-Sunnan (The Traditions). At-Tirmizhi relates: “I asked al-Bukhari if he confirms this hadith. He said: ‘It is authentic.'” At-Tirmizhi also says: “Jurists recognize that sadaqah should be taken out of any amount less than five ounces (awaq). One ounce (uqiyyah) equals forty dirhams. Five awaq equal 200 dirhams. Two hundred dirhams equal twenty-seven riyals equal 555 1/2 Egyptian piasters.”