From Sahih Bukhari (Hadith 6255):
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: Bukhari 6255