Abu Dawud Book 2, Hadith Number 1198.
Chapter : When should a traveller shorten his prayer.
Narated By Anas b. Malik : I prayed along with the Apostle of Allah (PBUH) four rak’ahs at the noon prayer at Medina and two rak’ahs at the afternoon prayer in Dhu al-Hulaifah. (at a distance of six miles from Medina).