When water is available, is it permissible to cleanse oneself with a stone or a tissue, and what is the evidence for that? And which is better: ritual washing with water (istinja') or cleansing with stones/tissues (istijmar)?

It is permissible to use stones or tissues for istijmar (cleaning oneself after relieving oneself), even if water is available. This is indicated by the Prophet Muhammad’s (peace be upon him) saying: "Whoever performs istijmar, let him do so an odd number of times." A condition for istijmar is to wipe at least three times or more until the area is clean, based on Salman al-Farsi’s statement: "The Prophet (peace be upon him) forbade us from cleaning ourselves with fewer than three stones." Cleaning oneself with water (istinja') is better, as it removes the substance and its traces, and is more thorough in cleansing.