مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 982
Prayer
What may not be done during Prayer and what is permitted - Section 1
وَعَنْ عَائِشَةَ رَضِيَ اللَّهُ عَنْهَا قَالَتْ: سَأَلَتْ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَنِ الِالْتِفَاتِ فِي الصَّلَاةِ فَقَالَ: «هُوَ اختلاس يختلسه الشَّيْطَان من صَلَاة العَبْد»
‘A’isha said: I asked God’s Messenger about looking to the side during prayer and he said, “It is something which the devil snatches from a servant’s prayer.” (Bukhari and Muslim.)
Mishkat al-Masabih, 982مُتَّفَقٌ عَلَيْهِ (الألباني)