Hadith 658B1

قَالَ مَالِك : أَرَى وَاللَّهُ أَعْلَمُ أَنَّهُ لَا يُؤْخَذُ مِنَ الْمَعَادِنِ مِمَّا يَخْرُجُ مِنْهَا شَيْءٌ ، حَتَّى يَبْلُغَ مَا يَخْرُجُ مِنْهَا قَدْرَ عِشْرِينَ دِينَارًا عَيْنًا أَوْ مِائَتَيْ دِرْهَمٍ ، فَإِذَا بَلَغَ ذَلِكَ فَفِيهِ الزَّكَاةُ مَكَانَهُ ، وَمَا زَادَ عَلَى ذَلِكَ أُخِذَ بِحِسَابِ ذَلِكَ مَا دَامَ فِي الْمَعْدِنِ نَيْلٌ ، فَإِذَا انْقَطَعَ عِرْقُهُ ثُمَّ جَاءَ بَعْدَ ذَلِكَ نَيْلٌ فَهُوَ مِثْلُ الْأَوَّلِ يُبْتَدَأُ فِيهِ الزَّكَاةُ كَمَا ابْتُدِئَتْ فِي الْأَوَّلِ
Imam Malik, may Allah have mercy on him, said: What I know is that nothing should be taken from the wealth extracted from mines until its value reaches twenty dinars or two hundred dirhams. However, when this amount is reached, zakat should be taken from it, and if it is more than this, zakat should be taken according to its amount as long as income from the mine continues. And when the income stops and then starts again, zakat will also start again just as it was taken from the previous income.
Hadith Reference موطا امام مالك رواية يحييٰ / كتاب الزكاة / 658B1
Hadith Takhrij «شركة الحروف نمبر: 535، فواد عبدالباقي نمبر: 17 - كِتَابُ الزَّكَاةِ-ح: 8»