Bukhari

Hadith Commentary:
The objective of migration (hijrah) that existed during the era of the Messenger (sallallahu alayhi wa sallam) came to an end with the conquest of Makkah, because the entire Arabian Peninsula became the abode of Islam (dar al-Islam). In later times, however, when the pattern of Makkan life reemerged, the process of migration (hijrah) continued.
Furthermore, Islam and jihad also remain.
Therefore, allegiance (bay‘ah) can be taken for all of these.
By bay‘ah is meant an oath and affirmation that one will certainly remain steadfast upon it, and will never go against it.
There are many types of bay‘ah, which will be explained.

Hadith Commentary:
1.
According to the first hadith, the noble Companions (radi Allahu anhum) pledged allegiance (bay‘ah) to the Messenger of Allah (sallallahu alayhi wa sallam) at the occasion of the Battle of the Trench (Ghazwah Khandaq) that they would continue to strive in jihad as long as they lived.
The meaning of this pledge was that they would not flee from the battlefield, even if their lives were to end in this path.
In the second hadith, there is mention of taking the pledge from Hazrat Mujashi‘ (radi Allahu anhu).
He had accepted Islam at the time of the Conquest of Makkah.
At that time, there was no opportunity to migrate (hijrah) from Makkah, because Makkah had now become a land of peace (dar al-salam).
2.
The Messenger of Allah (sallallahu alayhi wa sallam) used to take the pledge for migration (hijrah) from those who accepted Islam before the Conquest of Makkah. From those who accepted Islam after that, the pledge for migration was not taken; rather, the pledge for jihad was taken from them, because jihad is obligatory until the Day of Judgment. However, if a person’s faith is in danger in a particular country, then it is necessary for him to migrate from there to a place where his faith is secure and there is no obstacle in acting upon its requirements.
And Allah knows best.