Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) said: 

The Prophet (peace and blessings of Allaah be upon him) specified particular places for ihraam, from which the person who wants to do Hajj or ‘Umrah has to enter ihraam, and it is not permissible to pass them until he enters ihraam, because passing them before entering ihraam is transgressing one of the limits of Allaah, and Allaah says (interpretation of the meaning): 

“And whoever transgresses the limits ordained by Allaah, then such are the Zaalimoon (wrongdoers)”

[al-Baqarah 2:229] 

“And whosoever transgresses the set limits of Allaah, then indeed he has wronged himself”

[al-Talaaq 65:1]

In al-Saheehayn and elsewhere it is narrated from Ibn ‘Abbaas (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) defined the meeqaat of the people of Madeenah as Dhu’l-Hulayfah; that of the people of Shaam (Syria) as al-Jahfah; that of the people of Najd as Qarn al-Manaazil; and that of the people of Yemen as Yalamlam. And he said: “And these meeqaats are for the people at those very places, and besides them for those who come thorough those places with the intention of performing Hajj and 'Umrah; and whoever is living within these boundaries can enter Ihram from the place he starts” 

Narrated by al-Bukhaari, 1524; Muslim, 1181. 

It is also narrated in al-Saheehayn from ‘Abd-Allaah ibn ‘Umar (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “The meeqaat for the people of Madeenah is Dhu’l-Hulayfah.” Narrated by Muslim, 1183. This is a statement which is in fact a command, but it appears in the form of a statement to emphasize that this is the way in which it should be done. 

It was narrated from ‘Aa’ishah (may Allaah be pleased with her) that the Prophet (peace and blessings of Allaah be upon him) defined the meeqaat of the people of Iraq as Dhaat ‘Irq. Narrated by Abu Dawood, 1739; classed as saheeh by al-Albaani in Saheeh Abi Dawood, 1531. 

In Saheeh al-Bukhaari (1531) it is narrated that the people of Kufa and Basra came to ‘Umar ibn al-Khattaab (may Allaah be pleased with him) and said, “O Ameer al-Mu’mineen, the Messenger of Allaah (peace and blessings of Allaah be upon him) stated that Qarn was the meeqaat for the people of Najd, but it is out of our way, and if we want to go to Qarn it is difficult for us. He said: “Take as your meeqaat a place that is parallel with Qarn on your usual route.”  

So the one who intends to do Hajj or ‘Umrah must enter ihraam from these meeqaats when he comes to them, whether he comes by land or sea or air. 

If he is coming by land he should stop there when he passes through if it is on his route, or he should stop at a place that is parallel with it if it is not on his route, and do what is required when entering ihraam, namely doing ghusl, perfuming his body and putting on the ihraam garments, then he should enter ihraam before leaving the place. 

If he is coming by sea and if the ship will stop when it is parallel with the meeqaat, he should do ghusl, put on perfume and put on the ihraam garments when it stops, then he should enter ihraam before it moves on. If it does not stop when it comes in line with the meeqaat, then he should do ghusl, put on perfume and put on the ihraam garments before it comes in line with the meeqaat, and enter into ihraam when it comes in line with it. 

If he is coming by air, he should do ghusl before getting on board the plane, and put on perfume and the ihraam garments before the plane comes in line with the meeqaat, then he should enter into ihraam just before it comes in line with the meeqaat; he should not wait until it is in line with it, because planes travel quickly and he will not have the opportunity, so if he enters ihraam before to be on the safe side, there is nothing wrong with that.