Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Answering your question, the late Saudi scholar, Sheikh Ibn Baz (may Allah have mercy on him) stated:
It is legally sanctioned to raise one’s hands during the supplications of the Witr Prayer. This is because it is similar to the supplications during times of affliction.
It is confirmed that the Prophet (peace be upon him) used to raise his hands while supplicating during times of affliction. (Al-Baihaqi)
Almighty Allah knows best.