Compiled by Imam Yahya ibn Sharaf al-Nawawi
عن أنس بن مالك قَالَ : قَالَ رسولُ اللهِ : « مَا بَالُ أقْوامٍ يَرْفَعُونَ أبْصَارَهُمْ إِلَى السَّمَاءِ في صَلاَتِهِمْ » ! فَاشْتَدَّ قَولُهُ في ذَلِكَ حَتَّى قَالَ : « لَيَنْتَهُنَّ عَنْ ذَلِكَ ، أَوْ لَتُخطفَنَّ أَبْصَارُهُمْ » ! . رواه البخاري
The Messenger of Allah (ﷺ) said, "How is it that some people raise their eyes towards the sky during As-Salat (the prayer)?" He stressed (this point) and added, "People must refrain from raising their eyes towards heaven in Salat (prayer), or else their sights will certainly be snatched away."[Al-Bukhari].