Conversation so far:
What lead up to Umar's death?
Whenever Muhammad felt any relief from his fatal sickness, he would inquire as to whether Usama's army had left for Syria yet.
What was usamas army doing?
CANNOTANSWER.
Are there any other interesting aspects about this article?
Abu Bakr and Umar resisted going under the command of Usama because they thought that he, who was 18 or 20 at the time, was too young to.
What was he too young to do?
to lead an army.
What happened as a result of him refusing to follow under him?
Now obey his orders and go.

Question: Who stated this?
Self-contained rewrite: Who stated now obey his orders and go as a result of Abu Bak and Usama refusing to follow under Usama?