Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said, “When the day of Friday arrives there are angels upon the door of every mosque to record those who arrive in order and when the Imam sits to deliver the sermon they close their scrolls and come to listen to the remembrance. The example of one who arrives earliest is like one who offers a fine camel, then the next is like one who offers a cow, then the next is like one who offers a ram, then the next is like one who offers a chicken, then the next is like one who offers an egg.”
(Sahih Bukhari 887, Sahih Muslim 850)
What is meant here by the sitting of the Imam?
The narration refers to the Imam coming in at the beginning, meaning the start of the khutbah as suggested and explained by Imaam al Maawardi (rh) and Allah Knows Best.