It is narrated from Sayyiduna Jubayr bin Mut'im (may Allah be pleased with him) that in none of the prayers on a journey did the Messenger of Allah (peace and blessings be upon him) have the adhan called; he only had the iqamah called, except for Fajr, for which he (peace and blessings be upon him) had both the adhan and the iqamah called.
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1522