The worst of the worst (major) sins are: to associate partners with Allāh, to lose hope of the aid of Allāh, to despair of the mercy of Allāh, and to feel safe from the plans of Allāh.
Al-Ṭabarī, Al-Tafsīr (Qurān 4: 31). Graded ṣaḥīḥ by Ibn Kathīr in his tafsīr of the same verse.
Everyone should have times when they sit alone, remember their sins, and thus seek [Allāh’s] forgiveness for them.
Aḥmad b. Ḥanbal, Al-Zuhd article 2038.
If you see a man amongst you having committed a sin, do not supplicate Allāh against him and do not revile him; instead, pray to Allāh to cure him and accept his repentance, for when we used to see a man die upon something good, we would have hope for him, and when we used to see a man die upon wrongdoing, we would fear for him.
Ibn Abī Al-Dunyā, Kitāb Al-Tawbah article 112.
A man once came to Ibn ʿAbbās and said, “I proposed to a woman and she refused to marry me, then someone else proposed to her and she wanted to marry him; so I became jealous and killed her. Can I repent from this?” [Ibn ʿAbbās] asked, “Is your mother alive?” He replied, “No.” So he said, “Repent (tawbah) to Allāh the mighty and majestic, and worship him as much as you can.” ʿAṭāʾ said, “So I went and asked Ibn ʿAbbās, ‘Why did you ask him if his mother was alive?’ He replied, ‘I do not know of any action that brings one closer to Allāh than dutifulness to the mother.'”
Al-Bukhārī, Al-Adab Al-Mufrad ḥadīth 4. Graded ṣaḥīḥ by Al-Albānī. See Ṣaḥīḥ Al-Adab Al-Mufrad, Chapter on Dutifulness to the Mother; Al-Ṣaḥīḥah 6:711 ḥadīth 2799.
A man does not become noble until he has two qualities: he is undesirous of what other people possess, and he pardons and overlooks them.
Ibn Ḥibbān, Rawḍatu Al-ʿUqalāʾ p167
Do not be hasty in praising people or blaming them, for perhaps what pleases you from a person today will displease you tomorrow, and perhaps what displeases you today, will please you tomorrow. Indeed, people change. It is Allāh who forgives the sins. And Allāh is more merciful to his servant the day he meets him than a mother who lays out a bed for her child in an empty patch of land and feels [the ground]: if there is a risk of being stung, it will be her instead of him (her child), and if there is a risk of being pricked by a thorn, it will be her instead of him.
Al-Bayhaqī, Shuʿab Al-Īmān article 6177, and others.