Before providing an answer to the foregoing question, it would be appropriate to explain the common and specific times of the obligatory daily prayers:
The specific time for Zuhr prayers begins when the sun starts declining towards the West and continues until one can offer four rak'ats of prayer. This time period is only for the Zuhr prayer.
The time for Asr prayer is when there is time enough for offering four rak'ats of prayer till sunset. This is the time in which only Asr prayers can be offered.
The time common between Zuhr and Asr prayers is when the specific time for Zuhr prayer ends until the specific time for Asr prayer begins. This is the time in which the Zuhr and Asr prayers can be offered together without there being a gap between the prayers.
The specific time for Maghrib prayer is from the beginning of Maghrib (dusk) until the time for offering three rak'ats of prayers has passed. In this specific time, only the Maghrib prayer can be offered. The time for 'Isha prayers is when there is time enough for offering four rak'ats till midnight. In this short time, only Isha prayers can be offered.
The time common between Maghrib and Isha prayers is when the specific time for Maghrib prayer ends until the specific time for Isha prayer begins. This is the time in which the Maghrib and Isha prayers can be offered together without there being a gap between them.
Sunnis are of the view that the time for Zuhr prayer starts from midday until the shadow of everything becomes the size of that thing. This is the special time for Zuhr prayer and that Asr prayer cannot be offered within this time limit. Thereafter the time for Asr prayer starts until sunset. This is the special time for Asr prayer and that Zuhr prayer cannot be offered in this time.  When it comes to the time for Maghrib prayer, it starts from sunset until the brightness in west disappears. This is the time in which Isha prayers must not be offered. When the brightness in the west disappears, the time for Isha prayer starts and continues until midnight. This is the specific time for Isha prayers and that Maghrib prayers cannot be offered in this time span.[1]
According to Sunni school of jurisprudence, it is not permissible to combine Zuhr and Asr or Mghrib and Isha prayers but they allow the two prayers to be combined during a journey or when one is fearful or when it is raining. Thus, in such circumstances, they combine the prayers in the same way as Shiites do.[2]
Shiites, however, believe that Zuhr and Asr prayers can be offered after midday one after the other provided the sequence has to be observed. That is to say, Asr and Isha prayers have to be offered after Zuhr and Maghrib prayers, respectively. There is no problem in combining the prayers in such a manner.[3]
Sunnis argue that it has been reported that the Prophet (S) performed Zuhr and Asr prayers separately creating a time gap in between the two prayers. But Shiites are of the view that this is simply a Sunnah (lit. tradition) and a practical tradition or normative conduct of the Prophet does not and cannot prove the necessity of this act; in fact, it can imply necessity, desirability and allowableness (ibaha) of something and if one has to apply the Prophet's normative conduct to necessity, he has to present a separate evidence.[4]
Shiites, however, not only maintain that Sunni stance concerning impermissibility of combining prayers is not defendable but they also have solid proofs for their own argument. Shiites make recourse to a number of narrations which clearly refer to combining the Zuhr and Asr prayers on the part of the Holy Prophet (S) himself.[5]
Some narrations found in Sunni sources are the following:
Ibn Abbas has said: "The Prophet of Islam (S) performed seven rak'ats together and eight rak'ats also together."[6]
Sa'eid bin Jubair reports from Ibn Abbas that he said: "The Messenger of Allah (S) observed the Zuhr and Asr prayers; Maghrib and Isha prayers together in Medina without being in a state of fear or in a state of journey." Abu Zubair said: “I asked Sa'id (one of the narrators) why he did that. He said: I asked Ibn 'Abbas as you have asked me, and he replied that he (the Holy Prophet) wanted that no one among his Ummah should be put to (unnecessary) hardship.”[7]
It has been reported in a narration that Ibn Abbas was asked to tell the reason why the Prophet (S) combined between two prayers and he replied, "The Prophet (S) did so only for convenience of the Ummah."[8]
These narrations have been reported by Malik and Ahmad bin Hanbal in their books. In addition, a lot of narrations have been reported from the Imams (AS) in this regard.
Imam Sadiq (AS) said: "When the sun declines, the time for Zuhr and Asr prayers has started, except that the Zuhr is performed before the Asr prayers."[9]
There is no doubt that the Prophet of Islam sometimes offered the prayers in three times and he used to say: "I do so for the convenience of my Ummah."
Imam Sadiq (AS) says in this regard: "The Messenger of Allah (S) combined Zuhr and Asr; Maghrib and Isha Prayers with one Adhan and two Iqamas."[10]
Elsewhere Imam Sadiq (AS) says, "The Messenger of Allah (AS) offered Zuhr and Asr prayers together without an excuse."
Umar said to the Prophet (S): "Has anything new come into being?"
The Prophet (S) said, "No, nothing new has been revealed about prayer but I want my Ummah to be at ease."[11]
In closing, it should be noted that based on these arguments, Shiite scholars believe in the permissibility of offering prayers in three times or else no one suggests that there is anything wrong with praying the individual prayers separately. Zuhr and `Asr prayers and Maghrib and 'Isha' prayers can be offered either combined or separately. The fact that this well-evidenced sunnah is not generally practiced by Sunnis does not make it inapplicable in our lives. It should also be noted that permissibility of performing prayers in three times does imply necessity of doing so.
Of course, there is a saying in Nahj al-Balaghah about performing prayers separately: "The Messenger of Allah - the peace and blessing of Allah he upon him and his descendants - likened it to a hot bath situated at the door of a person who bathes in it five times a day. Will then any dirt remain on him?"[12]
This saying of the Commander of the Faithful, does not imply the necessity of performing prayers at five separate times; rather it includes also praying three times a day in the sense that every prayer offered by an individual is like washing once whereas offering five prayers means washing oneself five times irrespective of whether he offers the five prayers at five separate times or in three times. In other words, Imam Ali (AS) has made reference to one of the effects or benefits of praying.

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