Maliks Muwatta Book 3, Hadith Number 12.
Section : The Adhan During a Journey and Without Wudu.
Yahya related to me from Malik from Nafi that on a journey Abdullah ibn Umar did no more than the iqama, except for subh, when he called both the adhan and the iqama. Abdullah ibn Umar used to say, “The adhan is for an imam whom people join.”