Sahih Bukhari, Chapter: Pilgrimage (Hajj)

Abdullah bin Umar used to send the weak among his family early to Mina. So they used to depart from Al-Mash'ar Al-Haram (that is Al-Muzdalifa) at night (when the moon had set) and invoke Allah as much as they could, and then they would return (to Mina) before the Imam had started from Al-Muzdalifa to Mina. So some of them would reach Mina at the time of the Fajr prayer and some of them would come later. When they reached Mina they would throw pebbles on the Jamra (Jamrat-al-Aqaba) Ibn 'Umar used to say, "Allah's Apostle gave the permission to them (weak people) to do so."