The Prophet (blessings and peace of Allah be upon him) said: “When the night comes from here [i.e., the east] and the day departs from here [i.e., the west] and the sun sets, then the fasting person may break his fast.” Narrated by al-Bukhaari (1954) and Muslim (1100)

An-Nawawi (may Allah have mercy on him) said: The fast ends and is completed with the setting of the sun, according to the consensus of the Muslims.

End quote from al-Majmoo‘ Sharh al-Muhadhdhab (6/304)

Ibn ‘Abd al-Barr said: The day during which the fast must be observed lasts from the break of dawn until the setting of the sun. There is consensus among the Muslim scholars on this point.

End quote from at-Tamheed (10/62)

What is meant by sunset is the complete disappearance of the disc of the sun; no attention is to be paid to the red afterglow that remains on the horizon. Once the entire disc has disappeared, then it has become permissible to break the fast.

Al-Haafiz Ibn Rajab said: This hadith indicates that the mere disappearance of the sun’s disc means that the time for Maghrib prayer has begun, and the fasting person may break his fast at that point. This is the consensus of the scholars, as was narrated by Ibn al-Mundhir and others.

Our companions, the Shaafa‘is and others said: No attention is to be paid to the deep red afterglow that remains in the sky after the disc of the sun has disappeared from view.

End quote from Fath al-Baari (4/352)

Shaykh al-Islam Ibn Taymiyah said:

When the disc of the sun has disappeared, at that point the fasting person may break his fast, and the time when offering supererogatory prayers is disallowed has come to an end. The deep red afterglow that may remain on the horizon does not affect any rulings at all.

End quote from Sharh ‘Umdat al-Fiqh (p. 169)

An-Nawawi said:

No attention is to be paid to the rays that remain after the sun has set completely; rather the time (for Maghrib prayer) has begun even though the rays remain.

End quote from Majmoo‘ Sharh al-Muhadhdhab (3/29)