Question: Is it bid’at to fold the hands below the navel in salat?
ANSWER
No, it is not bid’at, but sunnat. Hadrat Ali narrates:
In salat, it is sunnat [for men] to place the hands below the navel with the right hand over the left hand. (Abu Dawud, Imam-i Ahmad)

That the hands are folded below the navel is written in all fiqh books

It is written in Durr-ul-mukhtar in its section dealing with the 18th of the sunnat elements of salat:

Men place their hands below their navels with their right hands over their left hands. Hadrat Ali stated, “It is sunnat to fold the hands below the navel.” In salat, a man should place his right hand over his left hand below his navel and grip his left wrist using his thumb and little finger of his right hand. This is the preferred ruling.

It is written in the book Ni’mat-i Islam in its section dealing with the 5th of sunnat elements of salat:

“For a man, [it is sunnat] to place his hands below his navel with his right hand over his left hand.”

It is possible to find these statements in all fiqh books.