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.
Ibn Masʿūd and [the rest of] the Companions of the Prophet ﷺ used to say the qunūt supplication before bowing in the witr prayer.
Ibn Abī Shaybah, Al-Muṣannaf ḥadīth 6911, and others.
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.
Fulfil what Allāh has obligated you to do and you will be from the most devout of people, stay away from what Allāh has forbidden you and you will be from the most careful of people, and be satisfied with what Allāh has apportioned for you and you will be from the richest of people.
Hannād b. Al-Sarī, Al-Zuhd, article 1032.
Verily a man continues to be truthful until he is written with Allāh as a true believer (ṣiddīq); and he persists in truthfulness until there is not even the space of a needle for sinfulness left in his heart to settle. And a man lies and persists in lying until there is not even the space of a needle for goodness in his heart to settle.
Wakīʿ b. Al-Jarrāḥ, Al-Zuhd ḥadīth 398.
The victim of theft might continue to suspect and conjecture [about who stole from him] until he becomes worse [in sin] than the thief.
Al-Bukhārī, Al-Adab Al-Mufrad no. 1289. Its chain of transmission was graded ṣaḥīḥ by Al-Albānī in Ṣaḥīḥ Al-Adab Al-Mufrad no.974.
A man once came to ʿAbdullāh [ibn Masʿūd] – Allāh be pleased with him – and said, “We were once travelling when we met another group of travellers, so we asked them, ‘Who are you?’ They replied, ‘We are the believers!'” So [Ibn Masʿūd] said, “So why didn’t they just say, ‘We are from the people of Jannah?!'”
Abū ʿUbayd Al-Qāsim b. Sallām, Kitāb Al-Īmān, article 11. Shaykh Al-Albānī graded the chain of transmission of this and the following narration ṣaḥīḥ according to the standard of Bukhārī and Muslim.
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.
The people will not cease to be well as long as they take knowledge from their seniors. When they take it from their juniors and their bad people, they will be destroyed.
Ibn ʿAbd Al-Barr, Jāmiʿ Bayān Al-ʿIlm article 1057.
If the people were gathered on one plain, all of them believers, except for two unbelievers amongst them, they would join each other. And if the people were gathered on one plain, all of them unbelievers, except for two believers amongst them, they would join each other.
Ibn Baṭṭah, Al-ʾIbānah Al-Kubrā 1:455