From ‘Umar ibn al Khattaab  radi Allaahu ‘anhu who said.
The Messenger of Allaah sall Allaahu ‘alaihi wa sallam said:

“When the night approaches from here,
and the day goes away from here,
then let the fasting person break his fast.”

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The Shaykh Muhammad ibn Saalih al ‘Uthaymeen rahimahullaah commented:

The Ameer ul Mu’mineen (Chief of the Believers) ‘Umar ibn al Khattaab radi Allaahu ‘anhu informs that the Prophet sall Allaahu ‘alaihi wa sallam clarified the signs of the time at which the fasting person breaks his fast.

And they are three signs which are inseparable from each other:
•         The approach of the night from the east
•         The departure of the day from the west
•         The setting of the sun.

And the foundation of these signs is the setting of the sun because with it enters the night, which Allaah the Most High has made the limit for completion of the fast.

However the night appearing clearly from the east and the departing of the day from the west are (also) a proof for it, and follow on from it.

(taken from Sh Uthaymeen’s explanation of ‘Umdat-ul-Ahkaam
translated by Nasser ibn Najam)

From ‘Aaishah  radi Allaahu ‘anhaa who said:

There were some fasts from Ramadaan which I had to make up, so I was not able to make them until Sha’baan[1].

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The Shaykh Muhammad ibn Saalih al ‘Uthaymeen rahimahullaah commented:

‘Aaishah radi Allaahu ‘anhaa informs that there were some fasts from Ramadaan which were upon her; she had refrained from fasting (during those days) due to an excuse, by way of her menstrual periods or other than that.

So she delayed them until Sha’baan, then she fasted them before the next Ramadaan.

And she clarified her excuse for that – that she was unable to make them up easily and conveniently until Sha’baan, when it became essential to make them up.

[1] The month before Ramadaan the following year

(taken from Sh Uthaymeen’s explanation of ‘Umdat-ul-Ahkaam
translated by Nasser ibn Najam)

From Sahl ibn Sa’d as Saa’idee radi Allaahu ‘anhu who said that the Messenger of Allaah sall Allaahu ‘alaihi wa sallam said:

“The people will not cease to be upon goodness as long as they hasten to break the fast.”

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The Shaykh Muhammad ibn Saalih al ‘Uthaymeen rahimahullaah commented:

All goodness is contained in following the Islamic Legislation and in restricting oneself to it, without going to excess or falling short.

And in this hadeeth, Sahl ibn Sa’d radi Allaahu ‘anhu informs that the Prophet sall Allaahu ‘alaihi wa sallam told (us) that the people will continue upon goodness and righteousness in their religion if they restrict themselves to (staying within) the Islamically legislated limits for their fasting and their breaking the fast.

So be prompt with breaking the fast after the setting of the sun – which is the (legislated) limit for the fasting – just as Allaah, the Most High, said:

Then complete your fasting till the nightfall…
Soorah al Baqarah (2) aayah 187

And the night enters with the setting of the sun.

(taken from Sh Uthaymeen’s explanation of ‘Umdat-ul-Ahkaam
translated by Nasser ibn Najam)