I heard Kaʿb bin Malik narrating (when he did not join the battle of Tabuk): Messenger of Allah ﷺ forbade all the Muslims to speak to us. I would come to Messenger of Allah ﷺ and greet him, and I would wonder whether the Prophet ﷺ did move his lips to return to my greetings or not till fifty nights passed away. The Prophet ﷺ 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.  

عَنْ تَبُوكَ وَنَهَى رَسُولُ اللَّهِ ﷺ عَنْ كَلاَمِنَا وَآتِي رَسُولَ اللَّهِ ﷺ فَأُسَلِّمُ عَلَيْهِ فَأَقُولُ فِي نَفْسِي هَلْ حَرَّكَ شَفَتَيْهِ بِرَدِّ السَّلاَمِ أَمْ لاَ حَتَّى كَمَلَتْ خَمْسُونَ لَيْلَةً وَآذَنَ النَّبِيُّ ﷺ بِتَوْبَةِ اللَّهِ عَلَيْنَا حِينَ صَلَّى الْفَجْرَ