It is not known who gave John the authority to perform this baptismal rite of washing away sins and it seems no one else knew either except for maybe Jesus, and he wasn't saying ?
Holding the Aaronic priesthood meant John had the keys to preach repentance and baptize for the remission of sins. Jesus recognized John as having the authority to baptize, and when the Savior began his ministry, he came to John to have that ordinance performed.
Aaronic Priesthood is mentioned in the Bible.
What the Bible says about Aaronic Priesthood
(From Forerunner Commentary)
Exodus 29:10
The Aaronic priests were purified for service to God through the transferal of their sins to a bull. Similarly, when an Israelite presented a peace or a sin offering, he laid his hands upon the animal being offered, identifying himself with it and transferring his guilt to the animal. Thus, the animal was set apart by God through the laying on of hands.
Martin G. Collins
Basic Doctrines: The Laying On of Hands
Related Topics: Aaronic Priesthood | Animal Sacrifices | Laying On of Hands | Peace Offering | Sin Offering | Transferring of Guilt
Leviticus 9:22-24
They fell on their faces out of fear, reverence, respect, awe, and wonder from the knowledge that God was in their midst. The last verse shows that God accepted Aaron and the priests because everything was done according to His will.
John W. Ritenbaugh
Examples of Divine Justice
Related Topics: Aaronic Priesthood | Awe, Reverential | Divine Justice | Fear of God | God's Justice | Respect | Respect for God | Reverence | Reverential Awe | Wonder
Hebrews 7:24-28
The Aaronic priesthood—including the high priest—was just as sinful as the population that they were to be serving. In order for this to be corrected, it was necessary that the true High Priest be one of divine nature, perfect, and sinless. Jesus Christ was both deity and humanity, and He qualified—through His sinlessness, His offering of His life, and His compassion—to be High Priest for the entirety of humanity. The book of Hebrews points out these things: 1) that He was divine, 2) that He offered His perfect life in sacrifice, and 3) that His mercy qualified Him to be High Priest.
Aaron's sons attained to the priesthood simply by being born into the Aaronic line. Members of the church, though, become priests by means of regeneration, making us part of the Divine Family—and thus brothers and sisters of Jesus Christ.