Ibn Majah

Commentary:
(1)
Sā‘ is a unit of measurement whose quantity is five ratl and one third ratl.
In terms of kilograms, its amount is two kilograms and one hundred grams, and according to some, it is two and a half kilograms. (Mudd)
A quarter of a sā‘ is called a mudd; its quantity is five hundred and twenty-five grams.
For liquids, a sā‘ is approximately a little more than two liters, and a mudd can be understood as a quarter of that.

(2)
Mentioning this amount for ritual bath (ghusl) and ablution (wudu) does not mean that it is not permissible to use less or more water than this.
The purpose is merely to state an estimate,
so that water is not wasted unnecessarily, but rather, a small amount of water is used in such a way that complete cleanliness is achieved. However, in the case of zakat al-fitr and similar matters, it is not correct to give less than a sā‘ of grain.

Hadith Commentary:
Benefits and Issues:
Imam Muslim rahimahullah has presented the last hadith on the basis of its conformity and agreement with other hadiths,
even though the narrator has individually expressed a lack of confidence in it.
From this, it is also understood that Imam Muslim rahimahullah did not consider it correct to distrust the narration of Abu Rayhanah from Safinah the Companion due to old age.

English Commentary:
1:
A mudd is equivalent to one ratl and one-third of a ratl, and a sa‘ is four mudd, which according to present-day weights is approximately two and a half kilograms.

2:
In Muslim, there is also a narration of the Prophet sallallahu alayhi wa sallam performing ritual bath (ghusl) with a "farq" of water. A farq was a vessel that held about seven kilograms of water. In one narration, it is even mentioned that ‘A’ishah radi Allahu anha and the Prophet sallallahu alayhi wa sallam would both perform ritual bath (ghusl) from a single farq of water. All of this depends on a person’s different circumstances and the bodies of different people. The same person may use more or less water due to cold or heat, or due to greater or lesser impurity of the body. Likewise, some people have large and broad bodies, while others are short and thin; accordingly, the need for water varies. In any case, one should not waste water unnecessarily beyond what is needed.