Prayer: Establishing the Time for Maghrib Prayer
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 722
If a person doubts whether the sun has set and deems it probable that it is hidden behind mountains, buildings, or trees, he must not perform the maghrib prayer before the redness of the sky in the east, which appears after sunset has passed overhead. Even if one does not have such a doubt, he must, based on obligatory precaution, wait until the aforementioned time.