Prayer: Mistakenly Starting Isha Before Maghrib
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 725
If before performing the maghrib prayer one inadvertently engages in performing the ʿishāʾ prayer and realises during the prayer that he has made a mistake, in the event that he has not performed the rukūʿ of the fourth rakʿah, he must change his intention to the maghrib prayer, complete the prayer, and then perform the ʿishāʾ prayer. However, if he has performed the rukūʿ of the fourth rakʿah, he can complete the ʿishāʾ prayer and then perform the maghrib prayer.