What is the ruling regarding the Dhuhr prayer if the worshiper doubts whether he performed istinja' (purification after defecation/urination) or not, and then ascertains after the prayer that he did not perform istinja'?

Istinja' (cleaning oneself after relieving oneself) before Wudu' (ablution) is not a condition for its validity. Some scholars hold that Wudu' is valid even before Istinja', considering Istinja' merely the removal of impurity, which is not a condition for the validity of purification (Taharah). If Istinja' is not performed out of ignorance, then that does not invalidate the prayer.