Al-Ghaafir 40:55

So be patient, for Allah’s promise is true. Seek forgiveness for your sin[13], and glorify your Lord with His praise evening and morning.

[13] Like all other prophets and messengers, Prophet Muhammad (ﷺ) was infallible of sins, especially concerning what they conveyed from Allah. However, flaws or mistakes could happen with prophets, like all other human beings. The examples include the Prophet’s response to the blind man (80:1-10); Jonah’s departure from his people without Allah’s permission (21:87-88); and David’s suspicion and fear of the two men who scaled the wall of his chamber (38:21-25). If the Prophet (ﷺ) himself is urged to seek Allah’s forgiveness, then the believers are far more in need of seeking Allah’s forgiveness.

So be patient, [O Muḥammad]. Indeed, the promise of Allāh is truth. And ask forgiveness for your sin[1375] and exalt [Allāh] with praise of your Lord in the evening and the morning.

[1375] What is intended is "fault" or "error" in judgement, since all prophets were protected by Allāh from falling into sin. The implication is that all believers should seek forgiveness for their sins.

55. So be patient (O Muhammad صلى الله عليه وسلم). Verily, the Promise of Allâh is true, and ask forgiveness for your fault[3] and glorify the praises of your Lord in the ‘Ashî (i.e. the time period after the midnoon till sunset) and in the Ibkâr (i.e. the time period from early morning or sunrise till before midnoon) [it is said that, that means the five compulsory congregational Salât (prayers) or the ‘Asr and Fajr prayers].

[3] (V.40:55) See the footnote of (V.4:106).

Then have patience (O Muhammad). Lo! the promise of Allah is true. And ask forgiveness of thy sin, and hymn the praise of thy Lord at fall of night and in the early hours.