بُلُوغُ الْمَرَامِ
Bulugh al-Maram 248
The Book of Prayer
وَعَنْ جَابِرِ بْنِ سَمُرَةٍ - رضى الله عنه - قَالَ : قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ لَيَنْتَهِيَنَّ قَوْمٌ يَرْفَعُونَ أَبْصَارَهُمْ إِلَى اَلسَّمَاءِ فِي اَلصَّلَاةِ أَوْ لَا تَرْجِعَ إِلَيْهِمْ } رَوَاهُ مُسْلِمٌ . [1]
Narrated Jabir bin Samura (RA): Allah's Messenger (ﷺ) said: "Those people who raise their eyes to heaven while in Salat (prayer) should stop (doing so) or else their sight will not return to them." [Reported by Muslim].
Sunnah.com reference: Book 2, Hadith 118