Did Abu Bakr or Umar lead the Companions in prayer after the Messenger of Allah, may Allah's peace and blessings be upon him, commanded one of them to lead the people in prayer due to his severe illness before his passing?

The one who led the people in prayer during the Prophet's illness, peace and blessings be upon him, was Abu Bakr al-Siddiq, may Allah be pleased with him. 'Umar ibn al-Khattab also led them in prayer once at the beginning of the Prophet's illness, peace and blessings be upon him, when Abu Bakr was absent. The Companions understood from the Prophet's insistence on Abu Bakr leading the prayer that he was the most rightful successor after him, so they pledged allegiance to him.