Is it true that the Prophet used one and a half liters for ablution (wudu') and five liters for ritual bathing (ghusl), and is this considered a Sunnah or an obligation, or is the use of water subject to need and the level of purity?

The Prophet, peace and blessings be upon him, used to perform ablution with one mudd (0.7575 liters) and ritual bathing with one sa' (3.30 liters); this indicates economy in water usage.

It is obligatory to pour water over the limbs during ablution and ritual bathing; merely wiping is not sufficient. There is no specific quantity of water required; a small or large amount is sufficient as long as water flows over the limbs.

It is advisable to economize in water and avoid extravagance. However, it is not obligatory to adhere strictly to one mudd. Rather, one should wash their limbs three times when performing ablution, ensuring thoroughness by covering the entire limb with water. Washing more than three times is an act of wrongdoing, transgression, and injustice.