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

عن عاصم الأحول، عن عبد الله بن سَرْجِسَ رضي الله عنه قال: قلت لرسول الله صلى الله عليه وسلم : يا رسول الله، غفر الله لك، قال: «ولك». قال عاصم: فقلت له: أستغفر لك رسول الله صلى الله عليه وسلم ؟ قال: نعم ولك، ثم تلا هذه الآية: {واستغفر لذنبك وللمؤمنين والمؤمنات}.
[صحيح] - [رواه مسلم بدون زيادة "قلت لرسول الله - صلى الله عليه وسلم: يا رسول الله، غفر الله لك، قال: «ولك»" فرواها النسائي]

‘Āsim Al-Ahwal reported that ‘Abdullāh ibn Sarjis (may Allah be pleased with him) reported: I said to the Messenger of Allah (may Allah's peace and blessings be upon him): "O Messenger of Allah, may Allah forgive you." He said: "And you." ‘Āsim added: "I asked him: 'Did the Messenger of Allah (may Allah's peace and blessings be upon him) asked forgiveness for you?' He said: ‘Yes, and for you, and he recited this verse: {And ask forgiveness for your sin and for the believing men and believing women.} [Sūrat Muhammad: 19]"
[Authentic hadith] - [An-Nasaa’i - Narrated by Muslim]

Explanation

‘Abdullāh ibn Sarjis (may Allah be pleased with him) informed us that he supplicated Allah, the Almighty, to forgive the Prophet (may Allah's peace and blessings be upon him) and so the Prophet supplicated Allah to forgive him as well. ‘Āsim asked Al-Ahwal ‘Abdullāh ibn Sarjis (may Allah be pleased with him) about whether the Prophet (may Allah's peace and blessings be upon him) had actually asked forgiveness for him, and he replied: "Yes, and for you as well", citing the verse in which Allah, the Almighty, commands the Prophet, saying: {And ask forgiveness for your sin and for the believing men and believing women.} [Sūrat Muhammad: 19]