Muwatta Imam Malik Book of Prayer, Hadith 13

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."

وَحَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، أَنَّ أَبَاهُ، قَالَ لَهُ إِذَا كُنْتَ فِي سَفَرٍ فَإِنْ شِئْتَ أَنْ تُؤَذِّنَ وَتُقِيمَ فَعَلْتَ وَإِنْ شِئْتَ فَأَقِمْ وَلاَ تُؤَذِّنْ . قَالَ يَحْيَى سَمِعْتُ مَالِكًا يَقُولُ لاَ بَأْسَ أَنْ يُؤَذِّنَ الرَّجُلُ وَهُوَ رَاكِبٌ .