The Sunnah: Ablution with a mudd and the full ritual bath with a sa'

Question : Even if one uses less than that.

Answer : It is sufficient. If one is thorough with less than a mudd in ablution, or less than a sa' in the full ritual bath, it is sufficient, because the objective is the washing, and this measure is for the sake of excellence. However, it is required that it not be merely wiping.
If it is wiping, it does not count. The difference between washing and wiping is that in washing, the water drips from the body, whereas in wiping, it does not. Wiping is like wetting your hand and rubbing the limb; it is known that the water does not drip. Washing is when the water flows over the part being washed and drips, even if only a little. The important thing is that the water flows over the limb. So if someone takes a small amount of water and keeps covering his hand and wiping his body like this, that is not sufficient, because Allah the Exalted said, "So wash your faces and wipe your heads," and He, Glorified and Exalted is He, distinguished between washing and wiping. Similarly, regarding His saying, "So purify yourselves," the Messenger (peace and blessings be upon him) clarified it by his action, showing that it is washing. Therefore, it must be washing. But if one is thorough with less than the measure, there is no harm; it is permissible.