Why are the hands raised while saying the takbir in the third rak'ah of the Witr prayer?

Acts of worship are “tawqīfī”, meaning they are performed as commanded by Allah and as described by the Prophet Muhammad (peace be upon him). Raising the hands during the takbir before the qunut in the Witr prayer is based on certain narrations (Abu Yusuf, al-Āthār, 21 [100]; Zayla'i, Nasb al-Rāyah, 1/389-391; Mubārakfūrī, Tuhfat al-Ahwadhī, 2/567).

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu