Musnad Ahmad 32:1766

'Affan narrated to us, Aban al-'Attar narrated to us, Qatadah narrated to us, from Anas, that the Prophet (peace and blessings of Allah be upon him) said: "What is the matter with people who raise their eyes toward the sky during their prayer?" His speech regarding that became severe, until he said: "They must desist from that, or their sight will surely be snatched away."