Yahya related to me that Malik said, "I have heard from Nafi thatAbdullah ibn Umar used to see his son, Ubaydullah ibn Abdullah, doingvoluntary prayers on a journey,and he would not disapprove of it." (ES)
USC-MSA web (English) reference: Book 9, Hadith 26 (ES)
Arabic reference (ES): Book 9, Hadith 355 (ES)