Prayer: The Obligatory Sequence of Joint Prayers
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 742
One must perform the ʿaṣr prayer after the ẓuhr prayer, and the ʿishāʾ prayer after the maghrib prayer. If someone intentionally performs ʿaṣr before ẓuhr or ʿishāʾ before maghrib, the prayer is invalid.