On the prohibition against looking to the sky during the prayer

Riyad as-Salihin (The Meadows of the Righteous)

by Imam Nawawi

340. Chapter: On the prohibition against looking to the sky during the prayer

1754. Anas ibn Malik said, "The Messenger of Allah, may Allah bless him and grant him peace, said, 'Why on earth do some people raise their eyes towards heaven during their prayer?' His words on this subject grew stronger until he finally said, 'They should stop doing it or their eyesight will be taken away.'" [al-Bukhari]