From Sahih Muslim (Hadith 4675):
According to still another version of the tradition narrated by the same authority, and handed down through a different chain of transmitters, it has been reported that they (the Companions of the Holy Prophet) were reciting rajaz verses and the Messenger of Allah (ﷺ) was (reciting) with them. And they were chanting:O God, there is no good but the good of the Hereafter. So help Thou the Ansar and the Muhajirs. Shaiban substituted" So forgive Thou" for" So help Thou
Reference: Muslim 4675