Assalaamu alaykum. I recently learned that washing between the buttocks is obligatory in the obligatory Ghusl (ritual bath), I did not know that before. I did not know that the place between the buttocks is considered along with visible parts of the body. All I had been taught is that the water should reach every visible part of the body. So are all my previous prayers invalid then?
All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
If you are certain that you did not wash that place and that water did not reach it, then there is a difference of opinion among the scholars about the validity of your previous prayers. Some scholars, including Shaykh Ibn Taymiyyah are of the view that you are excused because of your ignorance and that you are not obliged to make up for any of the prayers. Please, refer to fatwa 344401.
The majority of the scholars, on the other hand, are of the view that you are obliged to make up for them.
Hence, you may act according to the view of Shaykh Ibn Taymiyyah if it is difficult for you to make up for them. Also, if the whole matter is just a doubt on your part (i.e. you are not absolutely certain that the water did not reach that place), then just ignore it and do not pay any attention to it.
Allah knows best.
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