Who was the young leader whom the Messenger of Allah, peace and blessings be upon him, appointed over the army, and what was the reason that prevented him from departing for battle?

In Safar of the 11th year of the Hijra, the Prophet, peace be upon him, prepared an army led by Usama ibn Zayd, may Allah be pleased with him, which included senior Companions. He ordered it to head towards Balqa' and Darum. The army halted at al-Jurf due to the Prophet's illness, and he passed away a few days later. After the Prophet's death, many advised Abu Bakr al-Siddiq, may Allah be pleased with him, not to dispatch Usama's army due to their need for it, but he vehemently refused, saying: "By Allah, I will not untie a knot tied by the Messenger of Allah, peace be upon him." The departure of this army served a great interest; it terrified the enemies, and as a result, many tribes returned to Islam. The army triumphed over the Romans and returned safe and victorious.