According to search-discovered classical Islamic scholarship, the concept of Niyyah (Intention) is a foundational principle governing the validity and reward of all human actions. Its authority stems from the universally accepted hadith narrated by 'Umar ibn al-Khattab, where the Prophet Muhammad ﷺ stated, "Verily, actions are but by intentions, and every man shall have only that which he intended." Imam al-Bukhari and other great scholars placed this hadith at the beginning of their works, signifying that intention is the prerequisite for any deed. Classical authorities like Imam al-Ghazali, in his 'Ihya Ulum al-Din', elaborated that Niyyah is the soul of an action, distinguishing a deed performed as worship ('ibadah) from a mere habit ('adah). It is the inner resolve of the heart to perform an act solely for the sake of seeking Allah's pleasure. While the Quran in verse 2:225 establishes accountability for the heart's deliberate earnings, this hadith provides the definitive framework, making Niyyah a cornerstone of Islamic jurisprudence, spirituality, and ethics.