I never saw anyone say "I seek Allah's forgiveness and turn to Him in repentance" more often than the Messenger of Allah ﷺ.
ما رأيت أحدا أكثر أن يقول: أستغفر الله وأتوب إليه من رسول الله ﷺ.
No references are available.
Nasai narrated it in Amal al-Yawm wal-Layla (454), and Ibn Sunni (364), and Ibn Hibban graded it sahih (928), all of them through Saeed bin Abdul-Aziz, from Ismail bin Ubaydullah bin Abu Muhajir, from Khalid bin Abdullah bin Husayn, who said, "I heard Abu Hurairah say," then he mentioned it.Its isnad is hasan because of Khalid bin Abdullah bin Husayn, for a group of narrators reported from him, and Ibn Hibban mentioned him among the trustworthy. Abu Dawud said, "He was the most sound-minded person of his time," so someone like him has his hadith deemed hasan as long as it does not contain anything objectionable.