Musar על יומא 170:15

And let one not think: "Do I not say every day: 'Forgive us, our Father, for we have sinned'?" This would avail if one considered this [lashon hara] a sin and took it upon himself to guard himself against it. For penance requires, at least, absolute regret [for the past] and also acceptance [for the future]. And, in our many sins, one who is habituated to lashon hara does not consider it a sin at all. And even on Yom Kippur, when one says: [Forgive us] "for the sin that we have sinned before you with lashon hara," he does not accept upon himself 'guarding' for the future. And even if we say that he does, is this not among the sins between man and his neighbor, for which Yom Kippur does not atone until he conciliates his neighbor? (see Yoma 85b). And, a man like this — has he not certainly harmed several tens of men with his tongue? He has hurt this man by verbal wronging, and that one with lashon hara, and the other, with "whitening of the face" and the like. About such a one our sages of blessed memory have rightly said that a man by his lashon hara magnifies sin until the heavens. Therefore, a man must set his heart and his mind to this, that he not be "entrapped" above in this sin. They have also said "If one speaks lashon hara, he and I cannot live [together] in the world, as it is written (Psalms 10:15): 'He who slanders his neighbor in secret, him will I cut down… him will I not abide.' Read it not: 'him will I not abide,' but 'with him' will I not abide."

Therefore, it is proper that everyone who reveres God should lessen his usual occupations and let his thoughts be calm, and let him fix hours, during the day and a night, when he can sit alone in his rooms and examine his ways and search them out, and arise even before the watches of the morning to occupy himself with the paths of repentance. And let him not do as in most cases where people fast, or rise early to pray, but they do not actually wage war against the sins in order to remove from themselves every ugly thing. For if a man prays and fasts, and confesses, and still clings to his former ways — this is not the path of repentance. But a man should arouse himself to do complete repentance during the Ten Days of Repentance, for this is a time when God accepts his plea and his prayer is heard, as it is said, "In an acceptable time have I answered thee, and in a day of salvation have I helped thee" (Is. 49:8). And our Sages, of blessed memory, said, "Seek the Lord while He may be found" (Is. 55:6) — these are the ten days that are between Rosh Hashana and Yom Kippur (Rosh Hashana 18a). And they also said that the Day of Atonement, together with repentance, atones. Therefore Scripture has warned us that we should repent and make ourselves pure before God, Blessed be He, in our ways of repentance. As it is said, "From all your sins shall ye be clean before the Lord" (Lev. 16:30). And then he will atone for us with this day, to purify us (Yoma 85b).

He who smites his companion and causes him pain, whether it be in money matters or through fraudulent words, there is no atonement for him unless he can appease his companion. And the Day of Atonement atones only for sins that are between man and God. But as for sins between him and his fellow man, he must first appease him (Yoma 85b). He who lifts his hand against his companion, even though he did not smite him, is called a wicked man (Sanh. 58b), and he must ask forgiveness of him: only thus can he do atonement.