Riyad as-Salihin 17:246

Anas (May Allah be pleased with him) said:

The Messenger of Allah (ﷺ) said to me, "Beware of looking around in Salat (prayer), because random looks in Salat are a cause of destruction. If there should be no help from it, it is permissible in the voluntary and not in obligatory Salat." [At-Tirmidhi] .