Musnad Ahmad 907:235

'Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from al-A'mash, from al-Musayyab ibn Rafi', from Tamim ibn Tarafa, from Jabir ibn Samurah, from the Prophet (peace and blessings of Allah be upon him), who said: "People must certainly cease raising their eyes toward the sky during prayer, or else they will not return to them."