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.
We used to narrate: Whoever faults his brother for a sin he has repented to Allāh from, Allāh will try him with that very sin.
Aḥmad b. Ḥanbal, Al-Zuhd p342 (Al-ʿIlmīyah ed., 1983).
A person has not been true to Allāh, the mighty and sublime, if he loves recognition (fame).
Aḥmad b. Ḥanbal, Al-Zuhd article 2240.
Everyone gets happy and gets sad, but you should turn your happiness into gratitude (to Allāh) and your sadness into patience (ṣabr).
Ibn Kathīr, Tafsir of Al-Ḥadīd: 23.
Learn knowledge, understand it and benefit from it (live by it); and do not study it in order to decorate yourself with it, for if you live long you will likely see a time when knowledge will be used for beautification like a man beautifies himself with his garments.
Ibn Al-Mubārak, Al-Zuhd wa Al-Raqāʾiq no. 1056.
If you see a man being obstinate, argumentative and impressed with his own opinion, then his loss is complete.
Abū Nuʿaym, ʿHilyatu Al-Awliyāʾ 5:228
Whoever has been given wealth, beauty, garments or knowledge and then is not humble will have severe repercussions on the Day of Resurrection.
Ibn Abī Al-Dunyā, Al-Tawāḍuʿ Wa Al-Khumūl ḥadīth 90.
Knowledge only comes from learning, and forbearance comes from trying to be forbearing, and whoever seeks what is good will be given it, and whoever avoids evil will be protected from it.
Abū Khaythumah, Kitāb Al-ʿIlm (Albānī edition) ḥadīth 114. Al-Albānī grades its chain of transmission ṣaḥīḥ, and states it has been reported from the Prophet ﷺ. See Al-Ṣaḥīḥah 342.
Wear what you wish and eat what you wish, as long as you avoid two practices: extravagance and haughtiness.
Ibn Abī Shaybah, Al-Muṣannaf ḥadīth 26601.
No man speaks well about another while not knowing it to be true except that he is also likely to speak badly about him without knowing it to be true. And two people do not accompany each other doing something that is not obedience to Allāh except that it is likely they will part from each other doing something that is not obedience to Allāh.
Abū Bakr Al-Daynūrī, Al-Mujālasah wa Jawāhir Al-ʿIlm 5:274 article 2118.