The Book of Prayer
Bulugh al-Maram #289
كتاب الصلاة
وَلِلتِّرْمِذِيِّ: عَنْ أَنَسٍ - وَصَحَّحَهُ - «إِيَّاكَ وَالِالْتِفَاتَ فِي اَلصَّلَاةِ, فَإِنَّهُ هَلَكَةٌ, فَإِنْ كَانَ فَلَا بُدَّ فَفِي اَلتَّطَوُّعِ»
at-Tirmidhi has reported and graded as Sahih (authentic) the narration of Anas (RA):
“Avoid looking around when you are engaged in prayer, for looking around is destruction. And if you must do it, do so in the voluntary prayers.”
وَلِلتِّرْمِذِيِّ : عَنْ أَنَسٍ - وَصَحَّحَهُ - "إِيَّاكَ وَالِالْتِفَاتَ فِي اَلصَّلَاةِ , فَإِنَّهُ هَلَكَةٌ , فَإِنْ كَانَ فَلَا بُدَّ فَفِي اَلتَّطَوُّعِ"
"Avoid looking around when you are engaged in prayer, for looking around is destruction. And if you must do it, do so in the voluntary prayers."
— Bulugh al-Maram, Hadith 289