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