Does delaying prayers until a time of necessity, without a valid excuse, invalidate them or merely incur sin?

Performing the prayer at a time of necessity is considered performance (adaa') not making up (qadaa'), and it is valid and fulfills the obligation. Whoever delays it without an excuse commits a sin, and whoever delays it due to an excuse does not commit a sin. Congregational prayer is obligatory for adult men and is not waived except for an excuse.