Small amounts of drinking is permissible whilst offering Nafl prayers according to the relied upon position of the Hanbali madhhab, because this was narrated from some of the Sahabah. Based on this, it is permissible to drink some water whilst one is in the middle of praying Tarawih. This is from the Mufradat (positions exclusive to the madhhab) of the Madhhab.
*Small amounts is not specified, and is considered by ‘Urf. So what generally constitutes a small amount is permitted, such as a few sips.
[See Sharh al-Muntaha and Kashaf al-Qina’]