What is the legal ruling on a doctor saying to the companion of a patient who invokes the Messenger of Allah, peace and blessings be upon him, "And who is Muhammad?" And does this count as apostasy that necessitates ghusl, along with seeking forgiveness and uttering the two testimonies of faith?

This phrase is not disbelief (kufr), and the questioner's intention behind it is clear: to clarify the truth by distinguishing between the مقام الربوبية (status of Lordship) and the مقام النبوة (status of Prophethood). This resembles the statement of Abu Bakr Al-Siddiq, may Allah be pleased with him, on the day the Messenger of Allah, peace and blessings be upon him, died: "Whoever among you worshipped Muhammad, may Allah's peace and blessings be upon him, then Muhammad has died. And whoever among you worshipped Allah, then Allah is Alive and does not die." And he recited Allah's saying: "And Muhammad is not but a messenger. [Other] messengers have passed on before him. So if he was to die or be killed, would you turn back on your heels [to disbelief]? And he who turns back on his heels will not harm Allah at all; but Allah will reward the grateful." Nevertheless, it is preferable to use another phrase that conveys the meaning without the possibility of offense, such as: "Say, 'I am only a man like you.'"