Sahih Bukhari Volume 4, Book 52, Hadith Number 193.

Narated By Anas : Whenever Allah’s Apostle attacked some people, he would never attack them till it was dawn. If he heard the Adhan (i.e. call for prayer) he would delay the fight, and if he did not hear the Adhan, he would attack them immediately after dawn. We reached Khaibar at night.