Hastening or Delaying Breaking the Fast?
Question

Is there any reward for delaying breaking the fast until after praying Maghrib?
Praise be to Allah.The sunnah is to hasten to break the fast
There is no reward for delaying breaking the fast, rather it is better to hasten to break the fast as soon as the sun has set ; this brings a more complete reward. 
Al-Bukhari (1957) and Muslim (1098) narrated from Sahl ibn Sa’d that the Messenger of Allah (peace and blessings of Allaah be upon him) said: “The people will continue to do well so long as they hasten to break the fast.” 
Abu Dawud (2353) narrated the same from Abu Hurayrah; his report includes the additional phrase, “… because the Jews and Christians delay it.” (Classed as hasan by al-Albani in Sahih Abi Dawud, 2353) 
Al-Nawawi said: 
“This hadith urges us to hasten to break the fast as soon as it is established that the sun has set. What this means is that the ummah will continue to be in a sound state and they will be fine so long as they continue to adhere to this Sunnah. If they delay it, that will be a sign that corruption has arisen among them.” 
With regard to the words, “… because the Jews and Christians delay it,” al-Tibi said: 
“This reason indicates that the basis of the true monotheistic religion is the opposite of what its enemies among the People of the Book do, and that agreeing with them leads to doom and destruction.”  
Muslim (1099) narrated that `Aishah (may Allah be pleased with her) was asked about a man among the Companions of Muhammad (peace and blessings of Allah be upon him) – ‘Abd-Allah ibn Mas’ud – who hastened to pray Maghrib and break the fast. She said: This is what the Messenger of Allah (peace and blessings of Allah be upon him) used to do. 
Al-Shafi`i said in al-Umm: “Hastening to break the fast is mustahabb .” 
Ibn Hazm said in al-Muhalla (4/380): “It is Sunnah to hasten to break the fast and to delay suhur, i.e., when the sun disappears beneath the horizon of the fasting person and no more.”
Why is it recommended to hasten to break the fast?
The scholars have mentioned a number of reasons why it is recommended to hasten to break the fast, including the following: 

To be different from the Jews and Christians
To follow the Sunnah
So as not to make the day longer and take from the night
It is kinder to the fasting person and gives him more strength for worship
To hasten to eat that which Allah has permitted to him, for Allah is generous and the one who is generous likes to see people enjoying his generosity. So He likes His slaves to hasten to enjoy that which Allah has permitted to them as soon as the sun sets.

When to break your fast
The scholars unanimously agreed that the time for that is when it is established that the sun has set, by seeing it with one's own eyes or by being informed of that by two witnesses of good character, or of one according to the correct opinion. This was the view of al-Hafiz. (See: Fath al-Bari, commentary on hadith no. 1957; al-Sharh al-Mumti’, 6/267)
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And Allah knows best.