The Messenger of Allah (ﷺ) used to perform Ghusl (ritual bath) from Janābah (major ritual impurity) using one Sā‘ of water, and he would perform Wudū’ (ablution) with one Mudd

It is prescribed to use water moderately for ablution and ritual bathing and not to use it wastefully, even if water is abundant.
It is recommended to use little water for ablution and ritual baths, as needed. This is the guidance of the Prophet (ﷺ).
The bottom line is to perform ablution and ritual baths thoroughly while observing the relevant Sunnah acts and etiquettes, without extravagance or stinginess. One should take into account the time, the abundance or shortage of water, and so on.
Janābah applies to anyone who has ejaculated or engaged in sexual intercourse. It was called as such because the one in a state of Janābah should keep away (Ijtināb) from prayer and other acts of worship until he is purified from it.
Sā‘: a well-known measure. It refers to the Prophetic Sā‘, which weighs 480 Miskals of good wheat, or 3 liters.
Mudd: a Shar‘i unit of measurement, which is the handfuls of a moderate person when he fills both hands and extends them. Mudd is a quarter of a Sā‘, by consensus among the scholars, and it equals 750 milliliters.