(6) The Book on Traveling
أَبْوَابُ السَّفَرِ
(60) Chapter: What Has Been Mentioned About Looking Around During Salat
(60) باب مَا ذُكِرَ فِي الاِلْتِفَاتِ فِي الصَّلاَةِ
Jami` at-Tirmidhi 588 
Sa'eed bin Abi Hind narrated from some of the companions of Ikrimah:
"The Prophet would glance during Salat" and he mentioned a similar narration.
حَدَّثَنَا مَحْمُودُ بْنُ غَيْلاَنَ، حَدَّثَنَا وَكِيعٌ، عَنْ عَبْدِ اللَّهِ بْنِ سَعِيدِ بْنِ أَبِي هِنْدٍ، عَنْ بَعْضِ، أَصْحَابِ عِكْرِمَةَ أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ يَلْحَظُ فِي الصَّلاَةِ ‏.‏ فَذَكَرَ نَحْوَهُ ‏.‏ قَالَ وَفِي الْبَابِ عَنْ أَنَسٍ وَعَائِشَةَ ‏.‏
Grade :  Hasan (Darussalam)
Reference : Jami` at-Tirmidhi 588
In-book reference : Book 6, Hadith 45
English translation : Vol. 2, Book 1, Hadith 588