Sahih Bukhari Volume 4, Book 56, Hadith 2944

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدٍ، حَدَّثَنَا مُعَاوِيَةُ بْنُ عَمْرٍو، حَدَّثَنَا أَبُو إِسْحَاقَ، عَنْ حُمَيْدٍ، قَالَ سَمِعْتُ أَنَسًا ـ رضى الله عنه ـ يَقُولُ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِذَا غَزَا قَوْمًا لَمْ يُغِرْ حَتَّى يُصْبِحَ، فَإِنْ سَمِعَ أَذَانًا أَمْسَكَ، وَإِنْ لَمْ يَسْمَعْ أَذَانًا أَغَارَ بَعْدَ مَا يُصْبِحُ، فَنَزَلْنَا خَيْبَرَ لَيْلاً‏

Narrated 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.