"I heard Imaam ash-Shaafi'ee saying, 'There are two types of innovations. One praiseworthy and the other one blameworthy, and so whatever is according to the Sunnah is praiseworthy and whatever opposes it is blameworthy.' He used as evidence the statement of Umar concerning the taraweeh prayer when he said, 'This is a good innovation (bid'ah).'" 
But its existence does not contradict a previous command or action. 
This is because the Messenger of Allaah (صلى الله علیه وسلم) encouraged standing in Ramadhaan and he himself prayed in the mosque and some companions prayed with him for a number of nights but then (he sal-Allaahu 'alayhe wa sallam) stopped so that it did not become obligatory.
So after the death of the Messenger of Allaah (صلى الله علیه وسلم), when this practice stopped, the Companions agreed to pray the night prayer in congregation because it was the revival of a Sunnah which the Messenger of Allaah (صلى الله علیه وسلم) himself practiced and which he commanded and encouraged others to do.
In light of this it is agreed upon that a Bid'ah Hasanah (Good Innovation) is a means of reward, but only for every action that is in accordance with all the principles and conditions of the sharee'ah. So by performing the deed (the good innovation) no established action is contradicted or opposed, for example minarets, religious institutions and travel lodge's, they did not exist in the first era. As by doing a good deed and by co-operating on good actions the rulings of the sharee'ah are completed.
 Shaykh al-Albaanee said:
"I say this statement is incorrect because the Messenger of Allaah (صلى الله علیه وسلم) performed the night prayer in congregation as the author himself has mentioned. Rather he (the Messenger of Allaah sal-Allaahu 'alayhe wa sallam) also used to encourage the people by saying:"Whoever stands in the night prayer with the Imaam then he will receive reward as if he stood the whole night."
The chain of this hadeeth is authentic and the people of the Sunans have also mentioned it and I have checked this in Salaat ut-Taraweeh (p.17). So how can taraweeh in congregation be called a novelty? So what is correct is that the words of Umar (radi-Allaahu 'anhu), "This is a good innovation" does not mean innovation in the sharee'ah because every innovation in the sharee'ah is misguidance. Rather he indicated and took the linguistic usage of innovation (bid'ah), i.e. a new thing that did not happen before. So without doubt the taraweeh prayer in congregation behind one Imaam was not well known during the period of Abu Bakr and Umar (radi-Allaahu 'anhumaa) and this is why it was addressed with the word bid'ah (linguistic innovation) and the reason for its recommendation was because there is an evidence from the religion for it. This is a summary of this issue which requires more detail and this is not the time for it. For details see my aforementioned treatise and Imaam ash-Shaatibee's Ei'tisaam (Naasir-ud-Deen)."
 Bukhaaree has narrated this from Umar that taraweeh in congregation is the Sunnah and its revival. I have checked it in Salaat ut-Taraweeh (p.47-48).