Shaykh ʿAbdullāh ibn ʿAbd al-ʿAzīz ibn ʿAqīl:

In the past, we did not know of anyone praying except twenty units of prayer [in Tarāwīḥ]. And all the people used to be upon this. Then some of the scholars spoke about this such as Shaykh ibn Bāz¹ and Shaykh ibn ʿUthaymīn² and they said the Sunnah is eleven units of prayer. [Then] the people acted upon their statement bit by bit until it became common in most of [the] mosques today.

¹ See Majmū al-Fatāwá al-Shaykh (11/322)

² See Majmū al-Fatāwá al-Shaykh (14/187)