Sahih Bukhari Volume 8, Book 79, Hadith 6257

حَدَّثَنَا ابْنُ بُكَيْرٍ، حَدَّثَنَا اللَّيْثُ، عَنْ عُقَيْلٍ، عَنِ ابْنِ شِهَابٍ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ عَبْدِ اللَّهِ، أَنَّ عَبْدَ اللَّهِ بْنَ كَعْبٍ، قَالَ سَمِعْتُ كَعْبَ بْنَ مَالِكٍ، يُحَدِّثُ حِينَ تَخَلَّفَ عَنْ تَبُوكَ، وَنَهَى، رَسُولُ اللَّهِ صلى الله عليه وسلم عَنْ كَلاَمِنَا، وَآتِي رَسُولَ اللَّهِ صلى الله عليه وسلم فَأُسَلِّمُ عَلَيْهِ، فَأَقُولُ فِي نَفْسِي هَلْ حَرَّكَ شَفَتَيْهِ بِرَدِّ السَّلاَمِ أَمْ لاَ حَتَّى كَمَلَتْ خَمْسُونَ لَيْلَةً، وَآذَنَ النَّبِيُّ صلى الله عليه وسلم بِتَوْبَةِ اللَّهِ عَلَيْنَا حِينَ صَلَّى الْفَجْرَ‏.

Narrated By 'Abdullah bin Ka'b : I heard Ka'b bin Malik narrating (when he did not join the battle of Tabuk): Allah's Apostle forbade all the Muslims to speak to us. I would come to Allah's Apostle and greet him, and I would wonder whether the Prophet did move his lips to return to my greetings or not till fifty nights passed away. The Prophet then announced (to the people) Allah's forgiveness for us (acceptance of our repentance) at the time when he had offered the Fajr (morning) prayer.