Praying the Isha prayer after midnight is considered “tenzihen makruh” according to the Hanafi school. This means it is disliked but not to the level of “tahrimen makruh,” meaning it is not close to being forbidden. The prayer is valid, but the reward is diminished due to being performed at an inappropriate time. (Merginani, al-Hidaya fi Sharh Bidayat al-Mubtadi, 1/124)
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