What is the ruling regarding the prayers I performed in the past, before I knew that the trace left on clothing from flatulence is impure?

The mere emission of wind (flatulence) leaves no trace on clothes or body, and it is not impure, so there is no need to purify anything it may have touched.

If a trace is found on the underwear, it is from stool or an impure liquid. One must perform istinja' (cleansing after defecation/urination) from it, purify whatever it has touched of the body, clothes, and prayer area, and perform wudu' (ablution) due to its emission.

Prayers that were performed with clothes upon which there was a trace of impurity, while one was ignorant of it, do not need to be repeated. This is the opinion of the majority of scholars.