Uttering the intention is not a condition for the validity of for ihram for Hajj or `Umrah. So, if one wore ihram and had the intention of `Umrah, there is nothing wrong with his `Umrah even if he did not declare (utter) the intention. Wearing ihram, uttering talbiyah and starting the rituals represent one’s intention.

However, it should also be clear that originally, the Muslim who intends to perform `Umrah or Hajj must not pass the miqat without entering ihram, i.e., declaring the intention (which can be done silently by wearing ihram, uttering talbiyah before or at the miqat and starting the rituals) to perform Hajj, `Umrah, or both of them.

So, in case one passed the miqat without wearing ihram and making talbiyah, then he has to return to the miqat and enter ihram. If he returned to his country after performing `Umrah, he has to slaughter an animal by sending its price to someone who can do so at Al-Haram, or slaughter it by himself if he will go there later. If he cannot afford to slaughter, he has to fast 10 days.

Allah Almighty knows best.