Mukhtasar Muslim (Abridged) 195 — Virtue of the adhan

Sayyiduna Anas bin Malik (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) would attack the enemy early in the morning and would listen attentively for the sound of the adhan. If he heard the sound of the adhan (from the opponents’ city), he would not attack them. Once, he heard a man saying "Allahu Akbar, Allahu Akbar," so he said, "He is a Muslim." After that, he heard him say, "Ashhadu an la ilaha illallah, ashhadu an la ilaha illallah," so he said, "O man, you have been saved from the Fire." When people looked, he was a shepherd of goats.