رياض الصالحينAbū Zakariyyā Muḥyī al-Dīn Yaḥyā ibn Sharaf al-Nawawī٣٤١ - باب كراهة الالتفات في الصلاة لغير عذر
On the authority of ʿĀʾishah (may Allah be pleased with her), who said: I asked the Messenger of Allah (ﷺ) about turning [the head] aside during the prayer, and he said: "It is a snatching by which the devil steals from the servant's prayer." Narrated by al-Bukhārī.
On the authority of Anas (may Allah be pleased with him), who said: The Messenger of Allah (ﷺ) said: "Beware of turning [your head] aside during the prayer, for turning aside during the prayer is ruin; but if it is unavoidable, then [let it be] in voluntary prayer, not in the obligatory one." Narrated by al-Tirmidhī, who said: "A good and sound hadith."