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." (ID)
USC-MSA web (English) reference: Book 9, Hadith 26 (ID)
Arabic reference (ID): Book 9, Hadith 355 (ID)