(21) باب مَنْ لَمْ يُسَلِّمْ عَلَى مَنِ اقْتَرَفَ ذَنْبًا وَلَمْ يَرُدَّ سَلاَمَهُ حَتَّى تَتَبَيَّنَ تَوْبَتُه، وَإِلَى مَتَى تَتَبَيَّنُ تَوْبَةُ الْعَاصِي
Narrated `Abdullah bin Ka`b:
I heard Ka`b bin Malik narrating (when he did not join the battle of Tabuk): Allah's Messenger (ﷺ) forbade all the Muslims to speak to us. I would come to Allah's Messenger (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.
Reference: Sahih al-Bukhari 6255
In-book reference: Book 79, Hadith 29
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 8, Book 74, Hadith 272