أَنَّ الكَبِيرَةَ مَا رُتِّبَتْ عَلَيْهِ عُقُوبَةً خَاصَّةً…
وَمَا نَهِيَ عَنْهُ فَقَطْ وَلَمْ يُعَيِّنْ لَهُ عُقُوبَةً خَاصَّةً فَهُوَ صَغِيرَة
A major sin is whatever has a specific punishment as a consequence [of that action]. Whatever has been prohibited but for which there has not been particularised a specific punishment, then it is a lesser sin.
(From the Explanation of Safaareeniyyah of Shaykh al Uthaymeen)