Sahih Bukhari
Shortening the Prayers (At-Taqseer)
Volume: 002 / Book Number: 020 / Hadith Number: 208
Narrated 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.