In Makkah, Rasulullah (sallallahu alayhi wa sallam) carried on his
effort of da'wat by going personally to each and every person.
Apparently, after reaching Madinah, this did not remain the work of Nabi (Sallallahu alaihi wa sallam).
Instead Nabi (Sallahu alaihi wasallam) worked from a centre. This he did only after he had prepared a special jamaa'ah (group) consisting of those capable of efficiently giving the Makki da'wat with its prerequisites.
The requirement of this work then made it necessary for Nabi (Sallallahu alaih wasallam) to remain and organise the effort from a centre and to take work from the companions. On the basis of this it was correct for Umar (Radhiallahu anhu) to remain in the centre, Madinah, after thousands of Allah's servants were prepared to go for Jihaad in order to elevate the name of Allah in the lands of Iran and Rome.
It thus became necessary for Umar (Radhiallahu anhu) to remain in
Madina to firmly organise the work of Da'watul Haq (Call to the Truth) and Jehaad fie Sabililtnh (Jihaad in the Path of Allah).
Malfoozat Maulana Ilyas page no 18.