If I wake up and go to the bathroom after praying fajr and notice najaasah on my back passage (none on underwear) even though I performed istinjā the last time I used the bathroom, is the prayer valid? This has been happening a couple of times and I can not explain why except maybe sweating at night

Ustadh AbdulAziz al-Haqqan

If it is on the back passage itself, what I know and Allah knows best is that the wudhu is not valid, as it is a must that you make istinja before wudhu in these cases.