Purity: Doubt About Ghusl After Prayer
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 386:
If after performing prayers a person who had become junub doubts whether he had performed ghusl or not, the prayers he performed are valid; however, for future prayers he must perform ghusl. If after prayers he has a minor occurrence, it is necessary for him also to perform wuḍūʾ; and if there is time, he must, based on obligatory precaution, repeat the prayers he had performed.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)