Physician, heal thyself
Attend to one's own faults, in preference to pointing out the faults of others.
The phrase alludes to the readiness and ability of physicians to heal sickness in others while sometimes not being able or willing to heal themselves. This suggests something of 'the cobbler always wears the worst shoes', that is, cobblers are too poor and busy to attend to their own footwear. It also suggests that physicians, while often being able to help the sick, cannot always do so and, when sick themselves, are no better placed than anyone else.
From the Bible, Luke 4:23 (King James Version):
And he said unto them, Ye will surely say unto me this proverb, Physician, heal thyself: whatsoever we have heard done in Capernaum, do also here in thy country.
The text is usually interpreted to mean that Jesus expected to hear the proverb said to him in Nazareth, and that the people there would expect him to work miracles in his hometown as he had in other places.