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 nisab of which is that one dirham will be obligatory on every forty."
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 1243
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا اسناد ضعيف لضعف الحارث الأعور