Praise be to Allah.
The time of fasting on which the Muslims are unanimously agreed and which they have followed from the time of the Prophet (blessings and peace of Allah be upon him) and his Companions until the present day is from the break of the true dawn until when the disk of the sun disappears completely beneath the horizon. This is indicated by the Qur'aan and Sunnah and definitive consensus of the Muslims.
Allah says (interpretation of the meaning): “then complete your Sawm (fast) till the nightfall [al-layl]” [al-Baqarah 2:187]. Al-layl [translated here as nightfall] in the language of the Arabs starts when the sun sets.
It says in al-Qamoos al-Muheet (1364): al-Layl: from the setting of the sun until the break of the true dawn or sunrise. End quote.
It says in Lisaan al-‘Arab (11/607): al-Layl: comes straight after the end of al-Nahaar (day), and starts when the sun sets. End quote.
Al-Haafiz Ibn Katheer (may Allah have mercy on him) said in his commentary on this verse:
The words of Allah “then complete your Sawm (fast) till the nightfall” imply that breaking the fast at the moment of sunset is a shar’i rule. End quote.
Tafseer al-Qur’aan al-‘Azeem, 1/517
In fact some commentators have pointed out that the use of the preposition ila (till) in this verse also implies hastening, because that preposition indicates reaching the end and achieving the goal.
Al-‘Allaamah al-Taahir ibn ‘Ashoor (may Allah have mercy on him) said:
Ila al-layl (till the nightfall) means until one achieves that goal and reaches the night, and the preposition ila is chosen to indicate that one should hasten to break the fast when the sun sets, because the preposition ila means that the purpose is achieved then, unlike the preposition hatta (until). What is meant here is to indicate that the fast is completed when the night begins. End quote.
Al-Tahreer wa’l-Tanweer, 1/181
All of that is supported by what is narrated in al-Saheehayn from Ameer al-Mu'mineen ‘Umar ibn al-Khattaab (may Allah be pleased with him) who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “When the night comes from here and the day departs from here, and the sun sets, then it is time for the fasting person to break his fast.” Narrated by al-Bukhaari (1954) and Muslim (1100).
In this hadeeth the coming of night from the east and the disappearance of the disk of the sun below the horizon are mentioned together, which is something that is well known, because darkness starts from the East as soon as the light of the sun disappears below the horizon. Al-Haafiz Ibn Hajar (may Allah have mercy on him) said:
The phrase “when night comes from here” means from the east, and what is meant is when darkness becomes discernible. In this hadeeth he mentioned three things, because even though they are interconnected in fact, they may appear not to be connected, and it may be thought that night is coming from the east when it has not yet come, because something is covering the light of the sun; the same is also true of the departure of day. Hence it is clarified by the words “and the sun sets”, as an indication that it is essential to confirm that night has come and day has gone, and that these events occur because of the setting of the sun and not for any other reason. End quote.
Fath al-Baari, 4/196
Al-Nawawi (may Allah have mercy on him) said:
The scholars said: each of these three things implies the other two and is interconnected with them. Rather he mentioned them together because a person may be in a valley and the like in such a way that he cannot see the setting of the sun, so he relies on the coming of darkness and the disappearing of daylight. End quote.
Sharh Muslim, 7/209
Al-Bukhaari (1955) and Muslim (1101) narrated that ‘Abd-Allaah ibn Abi Awfa (may Allah be pleased with him) said: We were with the Messenger of Allah (blessings and peace of Allah be upon him) on a journey and he was fasting. When the sun set, he said to one of the people: O So and so, get up and make saweeq for us (mix it with water so that we can drink it). He said O Messenger of Allah, why not wait till the evening? He said: Dismount and make saweeq for us. He said: O Messenger of Allah, why not wait till the evening? He said: Dismount and make saweeq for us. He said: It is still day. He said: Dismount and make saweeq for us. So he dismounted and made saweeq for them, and the Prophet (blessings and peace of Allah be upon him) drank it, then he said: When you see that night has come from here, then let the fasting person break his fast.
Al-Haafiz Ibn Hajar (may Allah have mercy on him) said:
This hadeeth indicates that it is mustahabb to hasten to break the fast and that it is not permissible to refrain from eating and drinking for any part of the night at all; rather as soon as it is established that the sun has set, it is permissible to break the fast. End quote.
Fath al-Baari, 4/197.
Moreover, the fact that there is consensus among the Muslims regarding breaking the fast and eating as soon as one hears the muezzin give the call to Maghrib prayer when the sun disappears is evidence that this is the truth and the one who goes against that is following something other than the way of the believers and has introduced something into the religion for which he has no proof or reports from the scholars.
Al-Nawawi (may Allah have mercy on him) said:
Maghrib comes immediately after the sun has set, and this is something on which there is consensus. Something concerning this was narrated from the Shi’ah to which no attention should be paid and which has no basis. End quote.
Sharh Muslim, 5/136
In fact, in many of the books of the Shi’ah it mentions that on which there is consensus among the Muslims concerning this matter. Some of them narrated from Ja’far al-Saadiq (may Allah have mercy on him) that he said: “When the sun sets, it becomes permissible to break the fast and it becomes obligatory to offer the prayer.” End quote.
Al-Baroojardi narrated from the author of al-Da’aa’im that he said: We narrated from Ahl al-Bayt -- may the blessings of Allah be upon them all -- that there was consensus concerning that which we learned from the reports from them, that the onset of night which makes it permissible for the fasting person to break the fast is the disappearance of the sun beneath the western horizon without any barrier which would prevent seeing it such as a mountain or wall and the like. When the disk disappears beneath the horizon, then the night has begun and it is permissible to break the fast. End quote.
Jaami’ Ahaadeeth al-Shi’ah, 9/165
To sum up: what are some of the Shi’ah do now, of delaying Maghrib prayer and the breaking of the fast until sometime after the sun has set is contrary to what is indicated by the Holy Qur'aan, the saheeh Sunnah of the Prophet and the consensus of the Muslims.
Moreover it is contrary to what they themselves narrated from their Imams!
And Allah knows best.