Sahih al-Bukhari 430
Sahih al-Bukhari · Book 8: Prayers (Salat) · To offer As-Salat (the prayer) in the camel yards (the places where the camels are stationed)
Narrated Nafi`:
حَدَّثَنَا صَدَقَةُ بْنُ الْفَضْلِ ، قَالَ أَخْبَرَنَا سُلَيْمَانُ بْنُ حَيَّانَ ، قَالَ حَدَّثَنَا عُبَيْدُ اللَّهِ ، عَنْ نَافِعٍ ، قَالَ رَأَيْتُ ابْنَ عُمَرَ يُصَلِّي إِلَى بَعِيرِهِ وَقَالَ رَأَيْتُ النَّبِيَّ صلى الله عليه وسلم يَفْعَلُهُ.
"I saw Ibn `Umar praying while taking his camel as a Sutra in front of him and he said, "I saw the Prophet doing the same."