وَمَنْ كَانَ لَهُ نِصَابٌ، فَاسْتَفَادَ فِي أَثْنَاءِ الْحَوْلِ مِنْ جِنْسِهِ: ضَمَّهُ إِلَى مَالِهِ، وَزَكَّاهُ بِهِ.
Translation: Whoever owns a nisab and acquires more of its kind during the year joins it to his property and pays zakat on it together.
If a person already owns a qualifying nisab and acquires additional property of the same class during the running year, the addition is joined to the original property for the annual assessment. A new independent year is not begun here for each increment.
The rule presupposes that the original qualifying amount and its annual period are in place; it should not be applied without qualification when the initial nisab was absent.