According to this question all bidah in religion is bad. But good things Hz. Umar said about people who were praying taraweeh with jamaah is known (couldn't find what exactly Hz. Umar said. I will give reference later). So, my question is if praying taraweeh with jamaah a good deed or bidah and therefore a bad deed?
To understand this, you have understand what bid'ah is. People use this statement of Umar (radiallahu anhu) in wrong sense to promote Bid'ah (And this is what all they got to hang on to).
Now, what is prohibited is innovation in Religion. We call it al bida'atush Shar'iyyah (Innovation in Religion). That means bringing something into religion something that the Prophet (salallahu alayhi wassalam) never did before.
There is another innovation, that is, innovation in language, speech, customs etc. and this is perfectly permissible (mubah) in Islam.
So, inventions are divided into two:
A. Innovating in 'aadaat (customs/habits/culture etc.): such as inventing innovations of speech, and this is permissible (mubaah), because the basic principle regarding 'aadaat (customs) is one of permissibility (ibaahah).
B. And innovating in the religion, then this is prohibited because the basic principle regarding it is one of Tawaquf (restriction to authentic texts). He (sallallaahu 'alayhi wa sallam) said, "Whoever invents (ahdatha) in this matter of ours what is not from it, then it is rejected. "[Bukhaaree, Muslim] And in a narration, "Whoever performs an action not in accordance with our matter, then it is rejected. "[Saheeh Muslim]
Having this in mind, let us analyze the bid'ah that Umar (radiallahu anhu) mentioned. Did Umar (radiallahu anhu) do something the Prophet (salallahu alayhi wassalam) never did?
No. It is to be noted here that the Prophet (sallAllaahu alayhi wa sallam) had led the Taraaweeh Prayer in congregation for a few days, but then discontinued it due to his fear that the people would think it obligatory. This condition remained throughout the Khilaafah of Abu Bakr (radiAllaahu anhu), until 'Umar (radiAllaahu anhu) restarted what the Prophet (sallAllaahu alayhi wa sallam) had already laid a precedent for.
It is said in commentary of Riyadus Saliheen :
Some people say that performing Tarawih in congregation is a Bid`ah (innovation in religion) because it was introduced in the reign of 'Umar (May Allah be pleased with him). But this is not correct because it is established that the Prophet (sallallaahu ’alayhi wa sallam) did not continue it out of fear that it will be made obligatory; otherwise he would have carried on with it. When the fear that this practice be made obligatory was over, 'Umar (May Allah be pleased with him) gave it the form of a Nafl prayer and revived the mode of performing it collectively, and thus fulfilled the desire of the Prophet (sallallaahu ’alayhi wa sallam). In spite of all these facts, it is still permissible for one to perform Tarawih individually in the late hours of the night. Since ordinary people are not capable of performing it individually, the step taken by 'Umar (May Allah be pleased with him) is perfectly correct. If this had not been done, the majority of the people will have remained deprived of the blessings and reward of Qiyam-ul-Lail, which would have been a great deprivation indeed.
Thus, he (Umar) used the word bid'ah in a purely linguistic sense, and not in the Shar'ee (Religious) sense that is used for blameworthy things.
This is the Sunni view.
(This is a Shia view)
taraweeh prayers are Mustahab prayers of Ramazan month and is accepted by both Shia and Sunni scholars. But according to Shia scholars there is no evidence in Hadith and Sunnah showing Mustahab prayer is allowed to be done in Jamaah (toghether) and should be performed individually. So doing it by Jamaah is Haram Bidah. According to Sahih Bukhari Umar was first one who ordered performing it with Jamaah and said this is a good Bidah. And he did not mention any reason for it. But according to Shia scholars he wanted to show he has the legislating power like prophet and to show this sometimes made such orders.
Also Imam Ali S.A. banned it later in his government.