Anas bin Malik (May Allah be pleased with him) said: The Messenger of Allah (PBUH) 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."
Commentary
Looking towards the sky during prayer disturbs the concentration in Salat and there is a stern warning against this bad habit. One can, however, do so after Salat, i.e., during one's supplications.