Dear Mufti Saheb
I do not made Istinja properly by mistake, and after having performed ghusal with all its Faraid and wajiabat. I found between my buttocks some little traces of Najaasat. Is my ghusal valid?
In the name of Allāh, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-raḥmatullāhi wa-barakātuh.
Since you have made Ghusal, there is no need for you to repeat your Ghusal. Generally in Ghusal the entire extended body is washed. It is possible you are merely suspicious of the traces of impurity.
And Allah Ta‘ala Knows Best
Mufti Luqman Hansrot
 (Bahisti Zewar, Zam Zam publishers,Vol 2, Pg 63)