The Gospel of Matthew, an early Christian text, is the first book of the New Testament and is believed to have been written between 80 and 90 AD. It narrates the life events and teachings of Jesus, a spiritual teacher revered and studied by religious communities such as Christianity, Gnosticism, the Essene community, etc.
Below is an excerpt from Matthew where Jesus explains the importance of self-observation and inner change, with a notable emphasis on observing thoughts and emotions within rather than worrying about physical extremes. He teaches the importance of looking at the minute details in daily actions and attitudes, looking into things like anger and resentments, and so on. Also note the emphasis on the need for real inner change, as opposed to “just putting your gifts on the altar” and make external sacrifices:
 Ye have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment:  But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire.  Therefore if thou bring thy gift to the altar, and there rememberest that thy brother hath ought against thee;  Leave there thy gift before the altar, and go thy way; first be reconciled to thy brother, and then come and offer thy gift.
AND Ye have heard that it was said by them of old time, Thou shalt not commit adultery:  But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
~ The Bible, Matthew 5: 21 – 24 & 27 – 28 (King James Version)