Toward the end of Safar in the 11th year after the Hijrah, a few weeks before his death, the Prophet ﷺ ordered an expedition into the lands of the Romans (the Byzantine Empire). Its target was al-Sham, the region of Greater Syria, and in particular Palestine. He told the Companions to gather their weapons, including Abu Bakr RA, Umar RA and many other senior Companions.
He chose Usama ibn Zayd RA to lead them. Usama was only 17 or 18 years old. He was the son of Zayd ibn Harithah RA, the freed slave whom the Prophet ﷺ had once adopted. Usama had been born and raised in the Prophet ﷺ's household, and his father Zayd had been killed by the Romans at Mu'tah.
Some people in Madinah murmured about the choice. They said Usama was too young, that he was not from the Quraysh, and that his father had been a freed slave. None of the senior Companions objected, but the talk spread in the city. The Prophet ﷺ gathered the people and answered the criticism. He said, "If you dislike his leadership, then remember that you also criticized the leadership of his father before him." He swore that Usama was worthy to lead, and said that he was the most beloved of people to him after his father.
Usama set out from Madinah on the last day of Safar. When the army was one or two days away, a messenger came to tell him that the Prophet ﷺ had fallen ill. Usama camped outside the city for a few days. He came back into Madinah and visited the Prophet ﷺ on the day of his death, one of the last people to see him.