Maliks Muwatta Book 3, Hadith Number 13.

Section : The Adhan During a Journey and Without Wudu.

Yahya related to me from Malik from Hisham ibn Urwa that his father 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 the adhan.”

Yahya said that he heard Malik say, “There is no harm in a man calling the adhan while riding.”