The Prophetic traditions are firmly established.
Supplicating for the sick:
On the authority of Ibn 'Abbas (may Allah be pleased with them both),
that the Messenger of Allah (peace and blessings of Allah be upon him) entered upon a man to visit him in his illness,
and he (peace and blessings of Allah be upon him) said:
"No harm shall come to you; it is a purification, Allah willing."
Related by al-Bukhari.