Sahih Al Bukhari Hadith Number 1105

Narrated Salim bin `Abdullah: Ibn `Umar said, “Allah’s Messenger (peace be upon him) used to pray the Nawafil on the back of his Mount (carriage) by signs facing any direction.” Ibn `Umar used to do the same.

18  Shortening the Prayers (At-Taqseer)
Hadith number 1105
Volume no 2
Chapter: 12 ( Whoever offered Nawafil not after the compulsory Salat but before it)