Muwatta Malik, Book 17, Hadith 583 (In-Book) ⓘ

Yahya related to me from Malik that he had heard that Umar ibn Abd al-Aziz wrote to his governor in Damascus about zakat saying, "Zakat is paid on the produce of ploughed land, on gold and silver, and on livestock." Malik said, "Zakat is only paid on three things: the produce of ploughed land, gold and silver, and livestock."

وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ ‏:‏ أَنَّ عُمَرَ بْنَ عَبْدِ الْعَزِيزِ، كَتَبَ إِلَى عَامِلِهِ عَلَى دِمَشْقَ فِي الصَّدَقَةِ ‏:‏ إِنَّمَا الصَّدَقَةُ فِي الْحَرْثِ وَالْعَيْنِ وَالْمَاشِيَةِ ‏.‏ قَالَ مَالِكٌ ‏:‏ وَلاَ تَكُونُ الصَّدَقَةُ إِلاَّ فِي ثَلاَثَةِ أَشْيَاءَ ‏:‏ فِي الْحَرْثِ وَالْعَيْنِ وَالْمَاشِيَةِ ‏.‏