What is the ruling on قطع in prayer?

If what is meant by "interruption" in prayer is abandoning it at times and performing it at other times, then this is one of the gravest major sins, and its perpetrator is in great peril. He must repent and make up the missed prayers according to the majority of scholars.

If what is meant is interrupting the prayer after having started it, then it is not permissible to interrupt a fard (obligatory) prayer except for an excuse, due to His Almighty's saying: "And do not invalidate your deeds." As for a supererogatory prayer, the Shafi'is and Hanbalis permitted interrupting it, though with dislike, while the Hanafis and Malikis forbade it.