It is narrated from Sayyiduna Ali (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "I have exempted you from paying zakat on horses and slaves, therefore you must pay zakat on silver in any case, the minimum amount of which is that one dirham will be obligatory on every forty."
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 1097
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد ضعيف لضعف الحارث الأعور