ʿAbdullāh b. Masʿūd – Allāh be pleased with him – said:

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.