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)