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]. (DE)
App reference (DE): Book 18, Hadith 246 (DE)
Arabic/English book reference (DE): Book 18, Hadith 1756 (DE)