Anas bin Malik (May Allah be pleased with him) said:
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]. (DE)
App reference (DE): Book 18, Hadith 244 (DE)
Arabic/English book reference (DE): Book 18, Hadith 1754 (DE)