Wikipedia
Istinja means cleaning away whatever has been passed from the urethra or anus with water, or toilet paper or both, the necessary here is to completely remove impurity.
In some difficult cases people who live in remote, isolated, impoverished areas where water scarcity, isolation in a desolate places, trips to the desert... etc., they can use "istijmar" as type of istinja by stones or any clean pieces From Nature, or anything else that is pure and can clean the impurity ( Najasat) such as tissue paper, apart from bones and dung.
The necessary here is to completely remove impurity najasa. this is to be done when something impure is emitted, of stools or urine. but if nothing has come out apart from passing wind, then it is not obligatory to do instinja.
The Muslim must not only do istinja with lota (vessel) for prayer, istinja' with water is obligatory every time something is discharged from the bowel and bladder, such as urine or faeces. but it is not necessary for the sleeper or one who passes wind to perform Istinja, because Istinja is only prescribed for the removal of impurity ( najasa), and there is no impurity in this situation.