The saheeh Sunnah states that the Zakaat on gold and silver is one quarter of one tenth, i.e., 2.5%. A similar rate applies to trade goods and currency nowadays.
Al-Bukhaari (1454) narrated from Anas (may Allah be pleased with him) that Abu Bakr (may Allah be pleased with him) wrote to him this document when he sent him to Bahrain: “This is the obligatory charity that the Messenger of Allah (blessings and peace of Allah be upon him) enjoined upon the Muslims and that Allah enjoined upon His Messenger:… for silver, one quarter of one tenth.”
Abu Dawood (1572) narrated from ‘Ali (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “If you have two hundred dirhams and one full year has passed, then five dirhams are due on them. You do not have to pay anything - i.e. on gold -- unless you have twenty dinars; if you have twenty dinars and one full year has passed, then half a dinar is due on them, and if the number increases then work it out on this basis.” Classed as saheeh by al-Albaani in Saheeh Abi Dawood.
Ibn Maajah (1791) narrated from Ibn ‘Umar and ‘Aa’ishah (may Allah be pleased with them both) that the Prophet (blessings and peace of Allah be upon him) used to take from every twenty dinars or more, half a dinar, and from forty dinars, one dinar. Classed as saheeh by al-Albaani in Saheeh Ibn Maajah.
Ibn Abi Shaybah narrated in al-Musannad (9966) with a jayyid isnaad from ‘Ali (may Allah be pleased with him) that he said: Nothing is due on anything less than twenty dinars; for twenty dinars, half a dinar is due; for forty dinars, one dinar is due. For anything more than that, it is to be worked out. Irwa’ al-Ghaleel, 3/291
These hadeeths indicate that the Zakaat on gold and silver is 2.5%, and there is scholarly consensus on this point.
It says in al-Mawsoo‘ah al-Fiqhiyyah (21/29-30):
The fuqaha’ are unanimously agreed that the threshold for gold at which Zakaat becomes due is twenty dinars. When it reaches that amount, one quarter of one tenth should be paid on it. End quote.
It says in Fataawa al-Lajnah al-Daa’imah:
What is required is to pay one quarter of one tenth on what you have of gold or silver or currency or trade goods, if all of that reaches the minimum threshold by itself or when added to whatever you have of other wealth that is subject to Zakaat, whether it is money or trade goods, and one year has passed. End quote. Fataawa al-Lajnah al-Daa’imah, 9/439
With regard to when the obligatory duty of Zakaat began, that was in Makkah before the Hijrah, then from the second year AH the definition of the rates and the detailed rulings were established.
Hence some scholars say that Zakaat was made obligatory in the second year AH.
Ibn Katheer (may Allah have mercy on him) said:
It is not far-fetched to say that the principle of obligatory charity (Zakaat) was enjoined at the beginning of the Prophet’s mission, as in the verse (interpretation of the meaning): “but pay the due thereof on the day of their harvest” [al-An‘aam 6:141]. As for the Zakaat in which there is a minimum threshold and set rates, that was introduced in Madeenah. End quote.
He also said:
The obligation of Zakaat came in the second year AH in Madeenah, according to what was stated by more than one. End quote. Tafseer Ibn Katheer, 7/164
Al-Haytami said in Tuhfat al-Muhraaj (3/209):
The obligation to pay Zakaat on one’s wealth came in the second year AH, after sadaqat al-fitr was introduced. End quote.