Muwatta Malik, Book 54, Hadith 1801 (In-Book)

Malik related to me from Abd ar-Rahman ibn Harmala from Amr ibn Shuayb from his father from his father that the Messenger of Allah, may Allah bless him and grant him peace, said, "One rider is a shaytan. Two riders are two shaytans, and three are a riding-party."

حَدَّثَنِي مَالِكٌ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ حَرْمَلَةَ، عَنْ عَمْرِو بْنِ شُعَيْبٍ، عَنْ أَبِيهِ، عَنْ جَدِّهِ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ الرَّاكِبُ شَيْطَانٌ وَالرَّاكِبَانِ شَيْطَانَانِ وَالثَّلاَثَةُ رَكْبٌ ‏" ‏ ‏.‏

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also referenced under: Book 54, Hadith 35 (USC-MSA)