Sahih Bukhari
Shortening the Prayers (At-Taqseer)
Volume: 002 / Book Number: 020 / Hadith Number: 205
Narrated Hafs bin 'Asim:
Ibn 'Umar went on a journey and said, "I accompanied the Prophet and he did not offer optional prayers during the journey, and Allah says: 'Verily! In Allah's Apostle you have a good example to follow.' " (33.21)