أَخْبَرَنَا عَمْرُو بْنُ عَلِيٍّ، قَالَ حَدَّثَنَا عَبْدُ الرَّحْمَنِ، قَالَ حَدَّثَنَا زَائِدَةُ، عَنْ أَشْعَثَ بْنِ أَبِي الشَّعْثَاءِ، عَنْ أَبِيهِ، عَنْ مَسْرُوقٍ، عَنْ عَائِشَةَ، - رضى الله عنها - قَالَتْ سَأَلْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم عَنْ الاِلْتِفَاتِ فِي الصَّلاَةِ فَقَالَ ‏"‏ اخْتِلاَسٌ يَخْتَلِسُهُ الشَّيْطَانُ مِنَ الصَّلاَةِ ‏"‏ ‏.‏

It was narrated that 'Aishah, may Allah (SWT) be pleased with her, said: "I asked the Messenger of Allah (PBUH) about looking here and there during prayer. He said: 'That is something that the Shaitan snatches from one's prayer.'" Translation Not Available

Book Ref: Sunan An Nisai Book 13 Hadith 1196
Web Ref:  Sunan An Nisai Vol 2 Book 13 Hadith 1197