It is in this very sense that the critical scholars have understood ʿUmar’s remark. ʿAbd al-Wahhāb al-Subkī says in Ishrāq al-Maṣābīḥ fī Ṣalāt al-Tarāwīḥ (1/168) of his Fatāwā collection:

Ibn ʿAbd al-Barr stated: ʿUmar instituted nothing here except what the Messenger of Allah ﷺ had already instituted, loved, and approved. The only reason the Prophet ﷺ did not persist in it was his fear that it might be made obligatory upon his community, for he was most kind and merciful to the believers. When ʿUmar learned this from the Messenger of Allah ﷺ, and understood that after the Prophet’s death no obligatory act could be added to or subtracted from, he re-established the practice for the people, revived it, and ordered it to be done. That took place in the year 14 AH, and it was a distinction Allah had stored up for him and by which He favored him. Abu Bakr was not inspired to do it—despite his greater overall excellence and precedence in every good—yet each of them has virtues uniquely his own.

Al-Subkī adds:
Had the practice not been sought after, it would have been a blameworthy innovation, like the so-called Raghaʾib prayer on the night of mid-Shaʿbān and the first Friday of Rajab. In that case it would have been obligatory to denounce it, whereas the invalidity of denouncing the tarāwīḥ congregation is something known in religion as a matter of necessity.

The eminent scholar Ibn Ḥajar al-Haytamī, in his legal opinion, says:
“Expelling the Jews and Christians from the Arabian Peninsula and fighting the Turks are not innovations, even though they were not carried out during the Prophet’s lifetime, because they were actions commanded by him ﷺ.” ⦗51⦘

As for ʿUmar’s statement about the tarāwīḥ prayer, “What a good innovation this is,” he meant innovation in the linguistic sense: something done without precedent, as in Allah’s words, “I am not a novelty among the Messengers.” It is not an innovation in the legal sense, for a legal innovation is misguidance, as the Prophet ﷺ declared. When some scholars divided innovations into “good” and “bad,” they were only speaking of linguistic innovation. Whoever said, “Every innovation is misguidance,” was referring to legal innovation.
Do you not see that the Companions—may Allah be pleased with them—and the Successors after them objected to giving the adhān for anything other than the five daily prayers, such as the two ʿEids, even though there was no specific prohibition? They likewise disliked touching the two Syrian corners of the Kaʿbah and praying immediately after completing the saʿy between Ṣafā and Marwah, by analogy with the prayer offered after ṭawāf. In the same way, whatever the Prophet ﷺ deliberately left undone despite the presence of a reason to act, his very omission becomes a sunnah, and to perform it would be a blameworthy innovation.
Our qualification “despite the presence of the reason” excludes matters such as expelling the Jews and compiling the Muṣḥaf, which were accomplished during his lifetime, and those he left undone because an impediment existed—such as gathering for tarāwīḥ—for a fully effective cause includes within it the absence of all impediments. ⦗52⦘