If you pass the Miqat without making the Ihram intention, return to the Miqat if possible; if not, enter Ihram immediately and sacrifice a sheep as fidyah.
The intention (niyyah) is an essential pillar (rukun) of Ihram — without it, you are not considered to be in the state of Ihram 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 Haji, Umrah, or both (Tamattu/Qiran).
If you realize you have passed the Miqat without making the intention, the correct course of action menurut jumhur ulama is to return to the Miqat and enter Ihram from there before proceeding. If you return and enter Ihram 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 Ihram immediately from wherever you are and pay a fidyah — which is to sacrifice a sheep in Mekkah.
It is important to distinguish between forgetting the niyyah entirely and simply being confused about the type of Haji. If you made a general intention for ibadah haji but forgot to specify the type, most scholars say your Ihram 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.
Nabi (shallallahu alaihi wa sallam) said: 'Actions are judged by intentions, and every person will get what they intended.'
Nabi (shallallahu alaihi wa sallam) said: 'Actions are judged by intentions, and every person will get what they intended.'
Source: Ibn Qudamah, Al-Mughni; al-Kasani, Badai al-Sanai
Nabi (shallallahu alaihi wa sallam) said: 'Actions are judged by intentions, and every person will get what they intended.'
Sahih al-Bukhari #1 (sahih)