[Ruling on those who pray Maghrib at sunset before a night star appears]

Answer (and Allah grants success): The matter is one of ijtihād. Al-Hādī (peace be upon him) and the scholars of the school have a known view; Imām Zayd ibn ʿAlī, Aḥmad ibn ʿĪsā, ʿAlī ibn Mūsā, the jurists, and Imām Yaḥyā ibn Ḥamzah have another view: that the first chosen time of Maghrib is the setting of the sun’s disk, known by its disappearing behind the veil. This is related from those imāms in Sharḥ al-Azhār.
It has been established among the scholars of the school and others that every mujtahid is correct. Thus, whoever follows the scholars of the school is correct, and whoever follows Zayd ibn ʿAlī, Aḥmad ibn ʿĪsā, and others is also correct.
On that basis, it is not permissible to slander or wound reputations over this, nor should one avoid praying behind such imāms—except in Maghrib itself: one should not pray Maghrib behind them—not due to any flaw in their uprightness or piety, but due to the difference regarding the entry of the time; for whoever believes the time has not yet entered may not pray Maghrib. As for ʿIshāʾ, it is permissible to pray behind such a person.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1