Musnad Ahmad 32:164

Yahya ibn Sa'id narrated to us, from Sa'id, from Qatadah, that Anas narrated to them that the Prophet, peace and blessings of Allah be upon him, said: "What is the matter with people who raise their eyes during their prayer?" He was severe regarding that, until he said: "They must desist from that, or their sight will surely be snatched away."