Anas bin Malik reported,
Anas reported that when Abu Bakr 
dispatched him to Bahrain, he wrote out this letter for him, "This is the obligatory charity which the Messenger of Allah ﷺ made binding upon the Muslims."He mentioned the letter in its full length, and among its contents was, "On silver, one-fortieth is due. If it amounts to only one hundred and ninety dirhams, nothing is due on it unless its owner wishes to give voluntarily."
Azami 3968
Sahihعن أنس
أن أبا بكر رضي الله عنه: كتب له هذا الكتاب لما وجهه إلى البحرين: هذه فريضة الصدقة فرض رسول الله ﷺ على المسلمين. فذكر الكتاب بطوله وجاء فيه: وفي الرقة ربع العشر، فإن لم تكن إلا تسعين ومائة فليس فيها شيء إلا أن يشاء ربها.
Bukhari related it in zakat (1454), from Muhammad bin Abdullah bin Muthanna Ansari, who said, "My father narrated to me," and he said, "Thumama bin Abdullah bin Anas narrated to me, that Anas told him," and he mentioned the letter in full.Al-riqqah, pronounced with a short "i" on the ra and a single, unshaddad qaf, means pure silver, whether minted or unminted.His saying, "If it is only one hundred and ninety," gives the impression that if the amount exceeds one hundred and ninety before reaching two hundred, sadaqah would be due on it. But that is not so. He only mentioned ninety because it is the last unit of ten before reaching a hundred, and when counting goes beyond single digits, it is structured in tens, just as it is structured in hundreds and thousands. So he mentioned ninety to show that there is no sadaqah on anything short of two hundred, and this is confirmed by the words, "There is no sadaqah on less than five awaq."End. See Fath al-Bari (3/321).