بُلُوغُ الْمَرَامِ
Bulugh al-Maram 244
The Book of Prayer
وَلِلتِّرْمِذِيِّ : عَنْ أَنَسٍ - وَصَحَّحَهُ - { إِيَّاكَ وَالِالْتِفَاتَ فِي اَلصَّلَاةِ , فَإِنَّهُ هَلَكَةٌ , فَإِنْ كَانَ فَلَا بُدَّ فَفِي اَلتَّطَوُّعِ } [1]
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."
Sunnah.com reference: Book 2, Hadith 113