Musnad Ahmad 27:694

'Abd al-Rahman narrated to us, from Sufyan, from 'Ata', from Kathir ibn Jumhan, who said: I saw Ibn 'Umar walking between al-Safa and al-Marwah, so I said: "Do you walk?" He replied: "If I walk, I have seen the Messenger of Allah (may Allah bless him and grant him peace) walking; and if I run, I have seen the Messenger of Allah (may Allah bless him and grant him peace) running."