Hadith: Did the Prophet (may Allah's peace and blessings be upon him) ask forgiveness for you? He said: Yes, and for you, and he then recited this verse: {...ask forgiveness for your sin and for the believing men and believing women...} [Sūrat Muhammad: 19]

عن عبد الله بن سَرْجس، قال: رأيتُ النبي صلى الله عليه وسلم وأكلتُ معه خُبْزا ولحْمًا، أو قال ثَرِيدًا، قال فقلتُ له: أَسْتَغْفَرَ لك النبي صلى الله عليه وسلم ؟ قال: نعم، ولك، ثم تَلا هذه الآية {وَاسْتَغْفِرْ لِذَنْبِكَ وَلِلْمُؤْمِنِينَ وَالْمُؤْمِنَاتِ} [محمد: 19] قال: ثم دُرْتُ خَلْفَه فنَظَرْتُ إلى خاتَم النُّبوة بين كَتِفَيْه، عند ناغِض كَتِفِه اليُسْرى، جُمْعًا، عَلَيْه خِيلانٌ كأمْثال الثَّآلِيلِ.
[صحيح] - [رواه مسلم]

‘Abdullāh ibn Sarjis reported: I saw the Prophet (may Allah's peace and blessings be upon him) and ate bread and meat with him - or he said Tharīd (bread soaked in soup). I said to him: "Did the Prophet (may Allah's peace and blessings be upon him) ask forgiveness for you?" He said: "Yes, and for you, and he then recited this verse: {...ask forgiveness for your sin and for the believing men and believing women...} [Sūrat Muhammad: 19] I then went behind him and saw the seal of prophet-hood between his shoulders on the top of his left shoulder, like a clenched fist with spots on it, resembling moles.
Sahih/Authentic. - [Muslim]

Explanation

In this Hadīth, the Companion ‘Abdullāh ibn Sarjis (may Allah be pleased with him) explains that he saw the Prophet (may Allah's peace and blessings be upon him) and ate bread and meat with him. His disciple then asked him: "Did the Prophet (may Allah's peace and blessings be upon him) ask forgiveness for you?" He answered in the affirmative and added that he even asked forgiveness for his disciple and all believers, since Allah, the Almighty commanded him to do so in the Qur’an: {...ask forgiveness for your sin and for the believing men and believing women...} [Sūrat Muhammad: 19] After that, ‘Abdullāh ibn Sarjis told his disciple that he went behind the Prophet (may Allah's peace and blessings be upon him) and saw the seal of prophet-hood between his shoulders on top of his left shoulder, like a clenched fist with spots on it, colored differently from the rest of the skin. This description does not contradict the other narrations that described the seal of prophet-hood as a collection of hairs or likened it to a pigeon's egg. All these narrations are correct and reconcilable.