I used to think that I have to do wudu after ghusl, I thought that the ghusl wasn't for complete purification, and that I had to do wudu after if I needed to pray. I have been informed recently that it is forbidden to do wudu after ghusl, so do I have to repeat all those prayers I prayed?

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No your prayers are valid. You didn’t know so that’s ok. The wudhu that you did was simply void.

But from now on, after Ghusl al-Janabah, don’t do wudhu. You have full purity with it. Ghusl al-Jum’a also gives you purity according to Ayatollah Sistani, but you can still do wudhu after it (it’s recommended).