Sahih Al Bukhari – Book of Characteristics Of Prayer

Volumn 001, Book 012, Hadith Number 717. 
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Narated By Anas bin Malik RA: The Prophet PBUH 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."