"There is a night in this month which is greater in excellence than a thousand months; Allah has enjoined an extra salah to be offered in the nights of this month. If a person performs a nafl act, out of his own free choice during this month, he will be entitled to receive the reward of a fard act performed in other months…" (Mishkat)
In an other tradition, Prophet Muhammad, Sall-Allahu alayhi wa sallam, has regarded the tarawih salah as a means of the forgiveness of sins. He has said:
"The one who observes the tarawih salah at night during Ramadan with complete faith and devotion only for the sake of the recompense of the hearafter, will have all of his previous sins forgiven by Allah." (Bukhari, Muslim)
Scholars have regarded the collective offerings of the tarawih salah as a compulsory sunnah having the kifiyah status. According to consensus of the Companions, the tarawih salah consists of twenty rak'ahs which are to be offered two at a time with one salutation and after every four rak'ahs a short pause of rest has to be observed for the convenience of the people.The worshipper has the option to sit quiet during the pause after each four rak'ahs, or recite the tasbeeh, or offer a voluntary salah.
The witr salah is allowed to be offered collectively only during Ramadan.
It is sunnah of the Holy Prophet, Sall-Allahu alayhi wa sallam, to complete recital of the whole Quran, from the beginning to the end. One must understand that the tarawih salah is a compulsory sunnah of the Prophet, Sall-Allahu alayhi wa sallam, and has to be observed regularly until the end of the month of Ramadan, no matter whether the Quran has been recited once or more than once in the mosque.
The intention is to observe two rak'ahs of the tarawih salah, and thus to complete 20 rak'ahs with 10 salutations.
It is preferable to offer the witr salah after completion of the tarawih salah, but if the witr salah is offered before the tarawih salah, or after offering some of the rak'ahs of the tarawih salah, there is no harm.
If a late comer for the tarawih salah finds that the Imam has stood up for witr salah when he has yet to complete some of his tarawih rak'ahs he should join in witr congregation, and should complete his remaining rak'ahs of tarawih later in his own time.
It is desirable to observe the pause of rest after each four rak'ahs for as long as the time taken for observing the four rak'ahs; however, if the followers find it hard and inconvenient they may observe a shorter pause.
It is not valid to join in the tarawih congregation before first observing the fard salah of 'Isha, for the tarawih salah has to start after the 'Isha salah.
If a person has observed his fard salah in 'Isha in congregation, but has not observed the tarawih salah in congregation, he is permitted to offer his witr salah in congregation.