The Rites of Hajj (Kitab Al-Manasik Wa'l-Hajj)
Dawud :: Book 10 : Hadith 1899
Narrated Abdullah ibn Umar:
Kathir ibn Jamhan said: A man asked Abdullah ibn Umar between as-Safa and al-Marwah: AbdurRahman, I see you walking while the people are running (between as-Safa and al-Marwah)? He replied: If I walk, I saw the Apostle of Allah (peace_be_upon_him) running. I am too old.