Malik related to me from Abd ar-Rahman ibn Harmala from Amr ibnShuayb from his father from his father that the Messenger of Allah,may Allah bless him and grant him peace, said, "One rider is ashaytan. Two riders are two shaytans, and three are a riding-party."
USC-MSA web (English) reference: Book 54, Hadith 35
Arabic reference: Book 54, Hadith 1801