If you pass the Miqat without making the الإحرام intention, return to the Miqat if possible; if not, enter الإحرام immediately and sacrifice a sheep as فدية.
The intention (niyyah) is an essential pillar (ركن) of الإحرام — without it, you are not considered to be in the state of الإحرام regardless of what you are wearing. Niyyah is made in the heart, not necessarily spoken aloud, though some scholars recommend verbalizing it. You must specify whether you intend الحج, العمرة, or both (Tamattu/Qiran).
If you realize you have passed the Miqat without making the intention, the correct course of action عند جمهور العلماء is to return to the Miqat and enter الإحرام from there before proceeding. If you return and enter الإحرام from the Miqat, no penalty is due. However, if returning is genuinely not possible (e.g., on a flight that has already landed, or logistical impossibility), you should enter الإحرام immediately from wherever you are and pay a فدية — which is to sacrifice a sheep in مكة.
It is important to distinguish between forgetting the niyyah entirely and simply being confused about the type of الحج. If you made a general intention for الحج but forgot to specify the type, most scholars say your الإحرام is valid and you can specify later. To avoid these issues, make your intention clearly before reaching the Miqat, and if traveling by air, set an alarm for well before the estimated Miqat crossing time.
النبي (صلى الله عليه وسلم) said: 'Actions are judged by intentions, and every person will get what they intended.'
النبي (صلى الله عليه وسلم) said: 'Actions are judged by intentions, and every person will get what they intended.'
Source: ابن قدامة, Al-Mughni; al-Kasani, Badai al-Sanai
النبي (صلى الله عليه وسلم) said: 'Actions are judged by intentions, and every person will get what they intended.'
Sahih al-Bukhari #1 (sahih)