Narrated by Anas bin Malik(Radhiallaho anho): Allah's Messenger (sallallaahu 'alaihi wasallam) said "What is wrong with those people who look towards the sky during the prayer?" His talk grew stern while delivering this speech and he said, "They should stop (looking towards the sky during the prayer); otherwise their eyesight would be taken away."

Sahih Al Bukhari Vol 1 Number 717