Narrated Humaid:Anas bin Malik said, "Whenever the Prophet 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 they shouted 'Muhammad! By Allah, Muhammad and his army.' When Allah's Apostle 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."