What is the ruling on one who interrupts his prayer due to the obsessive whisperings (waswas) of changing intention, and repeats it many times?

The cure for obsessive thoughts (waswas) is to ignore them and pay them no mind. When you intend to pray, form the intention for the specific obligatory prayer in your heart, then begin immediately. Do not pay attention to any waswas that comes to you afterwards regarding the intention or its validity, and do not interrupt your prayer. These obsessive thoughts are from the trickery and cunning of Satan to spoil your worship and disrupt your life.