Musnad Ahmad 27:1989

Abdullah ibn al-Harith narrated to us, Dawud ibn Qays narrated to me, from Nafi’, from Ibn Umar, that he was on a journey, and a companion of his dismounted to perform the Witr prayer. Ibn Umar said: "What is the matter with you that you do not mount?" He said: "I am performing Witr." Ibn Umar said: "Is there not for you in the Messenger of Allah, may Allah bless him and grant him peace, a good example?"