Sahih al-Bukhari Book 10, Hadith 8
Anas bin Malik said, "Whenever the Prophet (ﷺ) went out with us to fight (in Allah's cause) against any
nation, he never allowed us to attack till morning and he would wait and see: if he heard Adhan he
would postpone the attack and if he did not hear Adhan he would attack them." Anas added, "We
reached Khaibar at night and in the morning when he did not hear the Adhan for the prayer, he (the
Prophet ) rode and I rode behind Abi Talha and my foot was touching that of the Prophet.
The inhabitants of Khaibar came out with their baskets and spades and when they saw the Prophet (ﷺ)
they shouted 'Muhammad! By Allah, Muhammad and his army.' When Allah's Messenger (ﷺ) saw them, he
said, "Allahu-Akbar! Allahu-Akbar! Khaibar is ruined. Whenever we approach a (hostile) nation (to
fight), then evil will be the morning of those who have been warned."
Abu Sa`id Al-Khudri told my father, "I see you liking sheep and the wilderness. So whenever you are with your sheep or in the wilderness and you want to pronounce Adhan for the prayer raise your voice in doing so, for whoever hears the Adhan, whether a human being, a jinn or any other creature, will be a witness for you on the Day of Resurrection." Abu Sa`id added, "I heard it (this narration) from Allah's Messenger (ﷺ)."
Allah's Messenger (ﷺ) said, "Whenever you hear the Adhan, say what the Mu'adh-dhin is saying.