When is it obligatory to perform Istinja (cleansing after defecation or urination), what are the cases in which it is not required, and does passing wind necessitate it?

Istinja' is the removal of impurities discharged from the two private parts (front and back) with water or a stone or similar materials. It is obligatory for anything that exits from them and is required when one intends to pray or perform any other act that necessitates ritual purity. Istinja' is not obligatory immediately; rather, it may be delayed until one intends to purify oneself or pray. According to the Hanbalis, istinja' is a condition for the validity of wudu'; wudu' is not valid before it. As for istinja' from wind, it is neither obligatory nor prescribed. Whoever performs it believing it to be prescribed has committed an innovation. And whoever's wudu' is nullified by the emission of wind is only required to perform wudu'.