Q: Why after Fard salaah do we keep our right hand on our heads?
A: It is not necessary to do so nor is it a part of the Salaah.
A Hadeeth mentions a Du’aa that can be recited after Namaaz. If the person recites this Du’aa then he will place or rub his right hand on the front of his head while reciting it.
Moulana Yusuf Laher