Why does The Reliance of the Traveller highlight 216 liters of water in the context of impurities?

*The Reliance of the Traveller*, The Classic Manual of Islamic Sacred Law 'Umdat al-Salik, by Ahmad ibn Naqib al-Misri (translated by Nuh Ha Mim Keller) states: > e1.4 Impure means it is neither purifying nor pure. (O: Namely: > > (1) less than **216 liters** of water (qullatayn) which is contaminated by filth (najasa), even when none of the water's characteristics (n: i.e. taste, color, or odor) have changed; > > (2) or **216 liters** or more of water when one of its characteristics of taste, color, or odor have changed (n: through the effect of the filth. As for the purity of water that has been used to wash away filth, it is discussed below at e14.14).) Here, "n:" indicates a "remark by the translator" and "O:" indicates an "excerpt from the commentary of Sheikh 'Umar Barakat". This quantity *216 liters* is mentioned many times (e1.7 through e1.16, e5.6, e9.1, e14.11, and probably more [I gave up enumerating]). It's a specific quantity, so it's reasonable to expect that it's derived from some Islamic source (and isn't just a best guess). I suspect it's equivalent to some less arbitrary quantity with another unit used during the time of the Prophet. **Question**: Why does *The Reliance of the Traveller* highlight 216 liters of water in the context of impurities? I also found it mentioned in a SeekersHub fatwa .