Why do Shia combine the prayers?
The Shi'ah acknowledge the five obligatory daily prayers. However, they frequently combine the Zuhr and 'Asr prayers by offering them consecutively during the time period defined by the start of Zuhr and the end of 'Asr. They also consider it permissible to combine the Maghrib and 'Isha prayers in a similar manner. This practice is in complete agreement with the Qur'an as well as authentic traditions from the Prophet (saw).
The Sunni schools of law (fiqh) - with the exception of the Hanafi school - allow the combining of the obligatory prayers (al-jam` bayn al-salatayn) in the case of rain, travel, fear, or other emergencies. The Hanafi school of law prohibits the combination of the daily prayers at any time, with the exception of the prayers at Al-Muzdalifa during the Hajj. The Maliki, Shafi'i, and Hanbali schools of law all agree on the permissibility of combining the prayers when one is travelling, but have differences of opinion on other reasons. The Shi'ah Ja'fari school of law states that one can combine the prayers without any particular reason.
Prayer times according to the Qur'an:
Imam Fakhr al-Din al-Razi, the famous Sunni commentator on the Qur'an, wrote regarding the verse quoted (Chapter 17, Verse 78):
"If we interpret the darkness (ghasaq) as being the time when darkness first appears then the term ghasaq refers to the beginning of Maghrib. On this basis, three timings are mentioned in the verse: ‘the time of noon, the time of the beginning of Maghrib and the time of Fajr'. This requires that noon be the time of Zuhr and `Asr, this time is shared between these two prayers. The time of the beginning of Maghrib is the time for Maghrib and 'Isha' so this time is also shared between these two prayers. This requires allowing the combining between Zuhr and `Asr and between Maghrib and 'Isha' at all times. However, there is proof to indicate that combining whilst at home without any excuse is not allowed. This leads to the view that the combining be allowed when travelling or when there is rain etc."
It is clear that the times of the obligatory prayers are only three:
1) The time of the two obligatory prayers, Zuhr (noon) and `Asr (afternoon), which is shared between the two.
2) The time of the two obligatory prayers Maghrib (dusk) and 'Isha' (night) which is also shared between the two.
3) The time for the Fajr (morning) prayer which is specific to it.
Did the Prophet (saw) himself combine prayers?
Ibn 'Abbas reported that the Messenger of Allah (may peace be upon him) observed in Medina seven (rak'ahs) and eight (rak'ahs), i. e. (combined) the noon (Zuhr) and afternoon (`Asr) prayers (eight rak'ahs) and the dusk (Maghrib) and night ('Isha') prayers (seven rak'ahs).
[Sahih al-Bukhari (English translation), volume 1, book 10, number 537; Sahih Muslim (English translation), Kitab al-Salat, Book 4, Chapter 100 Combination of prayers when one is resident, hadith no. 1522]
Now regarding Turbah: The Shia pray on natural rock, the ground, rugs made of palm tree leaves, or rugs made of grasslike material. We do not pray on a regular carpet. When a Shia prays, there is a piece of mud that is about the size of a matchbox. Many ignorant sunnis are ignorant as to why we use this in our prayers, saying that we pray to it. The piece of mud that the Shia pray on is from
the mud of Karbala, where our beloved Imam Hussein (a) was slaughtered by yazid's (la) army. This is not compulsory, as the mud can be from any place of the Earth, so long as it is clean, is not edible foods or materials that can be made into clothe that humans wear. Carpet and cement are not allowed because they may contain materials which are haram to prostrate one's head on. It is more humble and modest when one is between the hands of Allah (swt).
Another difference in Salah is the issue of hands. We Shia pray with our hands hanging down on our sides, the same way the Sunni Malikis do. Us Shia also don't say 'Amin' after the recitation of the first Surah in prayer is completed, as there is no proof to support the notion that this kind of behavior was performed by the Prophet (saw).