Abu Dawud Book 2, Hadith Number 1199.

Chapter : To pronounce the adhan while travelling.

Narated By Uqbah ibn Amir : I heard the Apostle of Allah (PBUH) say: Allah is pleased with a shepherd of goats who calls to prayer at the peak of a mountain, and offers prayer, Allah, the Exalted, says: Look at this servant of Mine; he calls to prayer and offers it and he fears Me. So I forgive him and admit him to paradise.