Sahih Bukhari
Characteristics of Prayer
Volume: 001 / Book Number: 012 / Hadith Number: 717
Narrated Anas bin Malik:
The Prophet 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 eye-sight would be taken away."