بُلُوغُ الْمَرَامِ
Bulugh al-Maram 243
The Book of Prayer
عَنْ عَائِشَةَ --رَضِيَ اَللَّهُ عَنْهَا-- قَالَتْ : { سَأَلْتُ رَسُولَ اَللَّهِ - صلى الله عليه وسلم -عَنْ اَلِالْتِفَاتِ فِي اَلصَّلَاةِ ? فَقَالَ : "هُوَ اِخْتِلَاسٌ يَخْتَلِسُهُ اَلشَّيْطَانُ مِنْ صَلَاةِ اَلْعَبْدِ } رَوَاهُ اَلْبُخَارِيُّ . [1]
Narrated 'Aishah (RA): I asked Allah's Messenger (ﷺ) about looking around during prayer and he said, "It is something which the devil snatches from a person's prayer." [Reported by al-Bukhari].
Sunnah.com reference: Book 2, Hadith 112