Sahih al-Bukhari 751
Sahih al-Bukhari · Book 10: Call to Prayers (Adhaan) · To look hither and thither in As-Salat (the prayer)
Narrated `Aisha:
حَدَّثَنَا مُسَدَّدٌ ، قَالَ حَدَّثَنَا أَبُو الأَحْوَصِ ، قَالَ حَدَّثَنَا أَشْعَثُ بْنُ سُلَيْمٍ ، عَنْ أَبِيهِ ، عَنْ مَسْرُوقٍ ، عَنْ عَائِشَةَ ، قَالَتْ سَأَلْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم عَنْ الاِلْتِفَاتِ فِي الصَّلاَةِ فَقَالَ " هُوَ اخْتِلاَسٌ يَخْتَلِسُهُ الشَّيْطَانُ مِنْ صَلاَةِ الْعَبْدِ " .
I asked Allah's Messenger (ﷺ) about looking hither and thither in prayer. He replied, "It is a way of stealing by which Satan takes away (a portion) from the prayer of a person."