put smb. off his stride

или throw smb. off (или out of) his stride

   пoмeшaть кoму-л., нapушить pитмичнocть чьeй-л. paбoты; выбить кoгo-л. из кoлeи

    These telephone calls put him completely off his stride and ruined a mornings work. He threw me out of my stride for a whole half day (E. S. Gardner)