Mishkat al-Masabih 3931
Anas said:
When the Prophet took us out on an expedition against a people he did not set out with us till the morning. He would look at them, and if he heard a call to prayer he refrained from them, but if he did not hear any he attacked them. We went out to Khaibar and reached them by night, but when the morning came and he heard no call to prayer he mounted. I mounted behind Abu Talha and my foot was touching God’s Prophet’s They came out to us with their large baskets and their spades, and when they saw the Prophet they said, “Muhammad, by God, Muhammad and the army," and went for refuge to the fortress. When God's Messenger saw them he said, “God is most great, God is most great. Khaibar has fallen. 1 When we alight in the courtyard of a people it is an evil morning for those who have been warned." 2 1.	 Or, 'May Khaibar fall. 
2.	 Cf. Al-Qur'an; 37:177.

(Bukhari and Muslim.)
وَعَنْ أَنَسٍ: أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَانَ إِذَا غَزَا بِنَا قَوْمًا لَمْ يَكُنْ يَغْزُو بِنَا حَتَّى يُصْبِحَ وَيَنْظُرَ إِلَيْهِمْ فَإِنْ سَمِعَ أَذَانًا كَفَّ عَنْهُمْ وَإِنْ لَمْ يَسْمَعْ أَذَانًا أَغَارَ عَلَيْهِمْ قَالَ: فَخَرَجْنَا إِلَى خَيْبَرَ فَانْتَهَيْنَا إِلَيْهِمْ لَيْلًا فَلَمَّا أَصْبَحَ وَلَمْ يسمَعْ أذاناً رِكبَ ورَكِبْتُ خلفَ أبي طلحةَ وَإِنَّ قَدَمِي لَتَمَسُّ قَدِمَ نَبِيُّ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: فَخَرَجُوا إِلَيْنَا بَمَكَاتِلِهِمْ ومساحيهم فَلَمَّا رأى النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالُوا: مُحَمَّدٌ واللَّهِ محمّدٌ والخميسُ فلَجؤوا إِلَى الْحِصْنِ فَلَمَّا رَآهُمْ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «اللَّهُ أَكْبَرُ اللَّهُ أَكْبَرُ خَرِبَتْ خَيْبَرُ إِنَّا إِذَا نَزَلْنَا بِسَاحَةِ قومٍ فساءَ صباحُ المُنْذَرينَ»