By definition, Eucharistic Adoration is the “extension” of the celebration of the Catholic Mass.
"Adoration is the first act of the virtue of religion. To adore God is to acknowledge him as God, as the Creator and Savior, the Lord and Master of everything that exists, as infinite and merciful Love. You shall worship the Lord your God, and him only shall you serve.” (CCC 2096)
Throughout history, the Church has had many prayerful Eucharistic practices. But perhaps the first direct and unmistakable reference to Eucharistic Adoration is found in the life of St. Basil (circa 375 AD). Basil is said to have divided the host into three parts when he celebrated Mass in the monastery. One part, he consumed. The second part, he gave to the monks, and the third, he placed in a golden dove suspended over the altar.