Sahih Bukhari Volume 002, Book 020, Hadith Number 208.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 20. Shortening The Prayers (At-Taqseer)
Narated By Salim bin Abdullah : Ibn 'Umar said, "Allah's Apostle used to pray the Nawafil on the back of his Mount (carriage) by signs facing any direction." Ibn 'Umar used to do the same.