Sahih Muslim Book - 6, Hadith - 1615

وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ أَبِى بَكْرِ بْنِ عُمَرَ بْنِ عَبْدِ الرَّحْمَنِ بْنِ عَبْدِ اللَّهِ بْنِ عُمَرَ بْنِ الْخَطَّابِ عَنْ سَعِيدِ بْنِ يَسَارٍ أَنَّهُ قَالَ كُنْتُ أَسِيرُ مَعَ ابْنِ عُمَرَ بِطَرِيقِ مَكَّةَ - قَالَ سَعِيدٌ - فَلَمَّا خَشِيتُ الصُّبْحَ نَزَلْتُ فَأَوْتَرْتُ ثُمَّ أَدْرَكْتُهُ فَقَالَ لِى ابْنُ عُمَرَ أَيْنَ كُنْتَ فَقُلْتُ لَهُ خَشِيتُ الْفَجْرَ فَنَزَلْتُ فَأَوْتَرْتُ. فَقَالَ عَبْدُ اللَّهِ أَلَيْسَ لَكَ فِى رَسُولِ اللَّهِ -صلى الله عليه وسلم- أُسْوَةٌ فَقُلْتُ بَلَى وَاللَّهِ. قَالَ إِنَّ رَسُولَ اللَّهِ -صلى الله عليه وسلم- كَانَ يُوتِرُ عَلَى الْبَعِيرِ.

It was narrated from Sa'eed bin Yasar that he said: "I was traveling with Ibn 'Umar on the road to Makkah. When I feared that dawn was about to break, I dismounted and prayed Witr, then I caught up with him. Ibn 'Umar said to me: 'Where were you?' I said to him: 'I was afraid that dawn would break, so I dismounted and prayed Witr.' 'Abdullah said: 'Do you not have an example to follow in the Messenger of Allah (s.a.w)?' I said: 'Of course, by Allah.' He said: 'The Messenger of Allah (s.a.w) used to pray Witr atop his camel."'