كتاب الصلاة
2  
The Book of Prayer
Bulugh al-Maram 244
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 } 1
Reference : Bulugh al-Maram 244
In-book reference : Book 2, Hadith 244
English translation : Book 2, Hadith 242
Hadith Text
Reference