Maliks Muwatta Book 20, Hadith Number 185.

Section : Going from Arafa to Muzdalifa.

Yahya related to me from Malik from Hisham ibn Urwa that his father said, “I was sitting with Usama ibn Zayd when some one asked him, ‘How did the Messenger of Allah, may Allah bless him and grant him peace, travel when he went from Arafa to Muzdalifa during the farewell hajj?’ and he replied, ‘He went at a medium pace, but when he found a gap (in the crowds) he speeded up.'”