Musnad Ahmad 134:1

Ali ibn Ishaq narrated to us, he said: Abdullah informed us, he said: Yunus informed us, from Ibn Shihab, he said: Ubayd Allah ibn Abdullah ibn Utbah ibn Mas'ud narrated to me, that a man from the companions of the Messenger of Allah (may Allah bless him and grant him peace) narrated to him, that he heard the Messenger of Allah (may Allah bless him and grant him peace) say: "When one of you is in his prayer, let him not raise his sight toward the sky, lest his sight be snatched away."