Imam al-Bukhari opens his entire collection with it, and scholars have called it a third of knowledge: innama al-a’mal bi al-niyyat — actions are but by intentions.
The narration
Umar ibn al-Khattab (may Allah be pleased with him) reported that the Prophet ﷺ said: “Actions are only by intentions, and every person will have only what they intended.” Recorded by al-Bukhari (1) and Muslim (1907).
What it changes
The hadith does not add a ritual; it re-prices the ones you already perform. The same commute, the same shift at work, the same cooking becomes worship or routine depending on a decision made silently before it starts. That is a heavy responsibility and an enormous mercy at once.