Abdullah bin Sarjas (may Allah be pleased with him) says:
"I came to Messenger of Allah ﷺ while there were people sitting in his company. I went around to the back of Messenger of Allah ﷺ (The narrator may have done this physically). Messenger of Allah ﷺ understood what I was trying to do. He removed the sheet (body wrap) from his back. I saw the place of the Seal of Prophethood between his two shoulders. It was like a cluster surrounded by til (moles) which appeared to be like a wart. I came before Messenger of Allah ﷺ and said to him. May Allah forgive you (or Allah has forgiven you, as is mentioned in Surah Fath, 'That Allah may forgive you, your former and your latter sins'). Messenger of Allah ﷺ replied: 'May Allah forgive you too'. The people said to me that Messenger of Allah ﷺ has made du'aa for your maghfirah (forgiveness). I replied: 'Yes, and for you too, because Allah Ta'aala has said: 'O Muhammad, seek forgiveness for yourself and the Mu'min males and females also"'. (That is why Messenger of Allah ﷺ asked for the forgiveness of Muslims).

حَدَّثَنَا أَحْمَدُ بْنُ الْمِقْدَامِ أَبُو الأَشْعَثِ الْعِجْلِيُّ الْبَصْرِيُّ، قَالَ: أَخْبَرَنَا حَمَّادُ بْنُ زَيْدٍ، عَنْ عَاصِمٍ الأَحْوَلِ، عَنْ عَبْدِ اللهِ بْنِ سَرْجِسَ، قَالَ: أَتَيْتُ رَسُولَ اللهِ ﷺ وَهُوَ فِي نَاسٍ مِنْ أَصْحَابِهِ، فَدُرْتُ هَكَذَا مِنْ خَلْفِهِ، فَعَرَفَ الَّذِي أُرِيدُ، فَأَلْقَى الرِّدَاءَ عَنْ ظَهْرِهِ، فَرَأَيْتُ مَوْضِعَ الْخَاتَمِ عَلَى كَتِفَيْهِ، مِثْلَ الْجُمْعِ حَوْلَهَا خِيلانٌ، كَأَنَّهَا ثَآلِيلُ، فَرَجَعْتُ حَتَّى اسْتَقْبَلْتُهُ، فَقُلْتُ: غَفَرَ اللَّهُ لَكَ يَا رَسُولَ اللهِ، فَقَالَ: وَلَكَ فَقَالَ الْقَوْمُ: أَسْتَغْفَرَ لَكَ رَسُولُ اللهِ ﷺ؟ فَقَالَ: نَعَمْ، وَلَكُمْ، ثُمَّ تَلا هَذِهِ الآيَةَ ?وَاسْتَغْفِرْ لِذَنْبِكَ وَلِلْمُؤْمِنِينَ وَالْمُؤْمِنَاتِ?..