Hadith 1754
Anas bin Malik (May Allah be pleased with him) said:
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].
In plain words
The Prophet sternly questioned why some people lift their eyes toward the sky in prayer, warning them to stop. It teaches us to keep the gaze lowered and humble while we stand before Allah.
Reference : Riyad as-Salihin 1754 In-book reference : Book 17, Hadith 244