Does one sin if he acts upon the opinion that permits performing the Isha prayer after midnight with karaha (disapproval), or is it obligatory to consider it forbidden without a valid excuse? And what are the evidences for each?

Scholars have differed concerning the ruling on praying 'Isha' after midnight. Some hold that it becomes a make-up (qada'), while others maintain that it is still performed on time (ada') but with sin incurred for the unjustified delay. The majority, however, view it as contrary to what is preferable (khilaf al-awla) but still performed on time (ada'). It is permissible to adopt the view of those who permit praying it after midnight, as long as it is a recognized opinion among people of knowledge. There is no obligation to adhere strictly to a specific madhhab.