Al-Hasan Al-Basri (rahimahullaah) said:

”Sincere repentance is that the slave (shows) regret for what has elapsed (of his sins) and is determined not to return to them.”

Al-Kalbee (rahimahullaah) said:

”It is to seek the forgiveness (of Allaah) with the tongue, being regretful in the heart and restrain the limbs (from committing the sins again).”

[Source: Madaarij As-Saalikeen 1/309-310]