Yahya related to me from Malik from Hisham ibn Urwa that hisfather said, "I was sitting with Usama ibn Zayd when some one askedhim, 'How did the Messenger of Allah, may Allah bless him and granthim peace, travel when he went from Arafa to Muzdalifa during thefarewell hajj?' and he replied, 'He went at a medium pace, but when hefound a gap (in the crowds) he speeded up.' "
USC-MSA web (English) reference: Book 20, Hadith 185
Arabic reference: Book 20, Hadith 884