Yahya related to me from Malik from Hisham ibn Urwa that hisfather said to him, "When you are on a journey you can, if you wish,call the adhan and the iqama, or, if you wish, the iqama and not theadhan."Yahya said that he heard Malik say, "There is no harmin a man calling the adhan while riding."
App reference: Book 3, Hadith 13
USC-MSA web (English) reference: Book 3, Hadith 13
Arabic reference: Book 3, Hadith 159