Question: What is the reason that we pray Zuhr-Asr and Maghrib-Isha prayers together, whereas for every Prayer there is a special time and the great leaders of Islam used to pray on the particular times. That is they used to pray five prayers are five different times a day?
Answer: There is no scope of argument in this matter that to establish Prayers five times a day and pray on the special times had been the practice of the Holy Prophet (‘s), the holy Imam (‘a) and the common Muslims at the early period of Islam and they used to generally pray five prayers at five different times.
Nothing can be said regarding this, but the argument is whether it is obligatory to keep a distance between two prayers (In the way many Mujtahids do) or it is a Recommended (mustahab) deed and is like other Recommended actions where a person has the choice to do it or not. They are not forced to pray together or separately, whether their praying of separately may be better?
The Shia scholars follow those traditions from which we come to know the actions of the Holy Prophet (‘s) and keeping those narrations in mind which have reached us through the Holy Imams (‘a) and following the literal meaning of the Holy Quran, in all the Islamic times the time gap between the two prayers is Recommended. And told the people that to keep the time distance and to recite and pray every Prayer in its special time is better and recommended, but then also one can leave the recommended acts, and the meaning of recommended is also this.
Undoubtedly by praying two Prayers together does not mean that we pray one Prayer in the time of other; for example if we pray Evening and Night prayers in the beginning of the night, it doesn’t mean that we have prayed Night at the wrong time; we have prayed both the Prayers in their proper times, because from sunset the time of both the Prayers begins till the midnight. (Except that after sunset the time required to pray three rakat (units) is the time of Evening Prayers and in the end approximately the time required to pray four rakat is the special time of Night and the remaining time is similar for both). And whenever we pray Night with Evening Prayers, meaning at the start of the night or pray Evening Prayer with Night Prayer at the last time of midnight, we pray both the Prayers in their specific time, but it is Recommended to pray Evening Prayer at the start of the night and Night Prayer after the time of Zawal-e-Shafaq and if a person doesn’t do like this, it means that he has just not performed a recommended act.
source : Religious Questions Answered: Logic for Islamic Rules by Ayt. Makarim Shirazi & Ayt. Jafar Subhani