What is the earliest instance of Eucharistic adoration?
The first recorded instance of Perpetual Adoration antedates Corpus Christi, and occurred at Avignon, on 11 September 1226.
Catholic devotion towards the Blessed Sacrament started in the 11th century:
Toward the end of the eleventh century we enter on a new era in the history of Eucharistic adoration. Until then the Real Presence was taken for granted in Catholic belief and its reservation was the common practice in Catholic churches, including the chapels and oratories of religious communities. Suddenly a revolution hit the Church when Berengarius (999-1088), archdeacon of Angers in France, publicly denied that Christ was really and physically present under the species of bread and wine. Others took up the idea and began writing about the Eucharistic Christ as not exactly the Christ of the Gospels or, by implication, as not actually there.
The matter became so serious that Pope Gregory VII ordered Berengarius to sign a retraction. This credo has made theological history. It was the Church's first definitive statement of what had always been believed and never seriously challenged. The witness came from the abbot-become-pope, whose faith in the Blessed Sacrament had been nourished for years in a Benedictine monastery.
Gregory's teaching on the Real Presence was quoted verbatim in Pope Paul VI's historic document Mysterium Fidei (1965) to meet a new challenge to the Eucharist in our day--very similar to what happened in the eleventh century.
From the eleventh century on, devotion to the Blessed Sacrament reserved in the tabernacle became more and more prevalent in the Catholic world. At every stage in this development, members of religious orders of men and women took the lead.
Feast of Corpus Christi
There was nothing startling, therefore, when Pope Urban IV, in the thirteenth century, instituted the feast of Corpus Christi. When establishing the feast, the Pope stressed the love of Christ who wished to remain physically with us until the end of time.
In the Eucharist, said the Pope, "Christ is with us in His own substance." For "when telling the Apostles that He was ascending into heaven, He said, 'Behold I am with you all days, even to the consummation of the world,' thus comforting them with the gracious promise that He would remain and be with them even by His bodily presence" (August 11, 1264).
Urban IV commissioned Thomas Aquinas to compose the Liturgy of the Hours for the feast of Corpus Christi, to be celebrated annually on the Thursday following Trinity Sunday.
Aquinas, like the Church, never separated the Eucharist as Sacrifice, Communion and Presence. But, with the Church, he also realized that without the Real Presence there would be no real sacrifice nor real communion. Aquinas assumed that God became man so He might offer Himself on Calvary and continue to offer Himself in the Mass. He became man that He might give Himself to the disciples at the Last Supper and continue to give Himself to us in Holy Communion. He became man to live in flesh and blood in Palestine and continue to live now on earth as the same Jesus Who died and rose from the dead and is seated at the right hand of His heavenly Father. - The History of Eucharistic Adoration: Development of Doctrine in the Catholic Church
The Catholic Encyclopedia has this to say on the subject:
No trace of the existence of any such extra-liturgical cultus of the Blessed Sacrament can be found in the records of the early Church. Christian Lupus, indeed, argues that in the days of St. Ambrose and St. Augustine it was customary for the neophytes to adore, for eight days following their baptism, the Blessed Sacrament exposed, but no sound proof is adduced. It first appears in the later Middle Ages, about the beginning of the thirteenth century. It certainly may be conjectured that such adoration was really connoted by the fact of reservation in the early Church, especially in view of the evident desire to have the Eucharist represent the unity and continuity of the Church, as it is unlikely that there should not be some continuation of the adoration evidently given to the Host at the Synaxis. But such conjecture cannot be insisted upon
in view of the remarkable fact that no trace of any such adoration is to be found in the lives of saints noted for their devotion to the Blessed Sacrament in Holy Communion; thus it is remarkable that St. Ignatius in "The Spiritual Exercises", when directing attention to the abiding presence of God with His creatures as a motive for awakening love says not a word of the Blessed Sacrament;
because of the practice of even the present-day Greek Church which, although believing explicitly in transubstantiation, has never considered Our Lord in the Blessed Sacrament "our companion and refuge as well as our food" (Thurston).
The slowness with which the Exposition of the Blessed Sacrament came into vogue, and the also slow development of the custom of paying visits to the Blessed Sacrament [Father Bridgett asserting that he had not come across one clear example in England of a visit to the Blessed Sacrament in pre-Reformation times (Thurston, ib.)], render it increasingly difficult to make out a case for any adoration, perpetual or temporary, outside the Mass and Holy Communion, as these various forms of devotion are closely linked together. Most liturgists rightly attribute the Exposition of the Blessed Sacrament and its special adoration to the establishment of the Feast of Corpus Christi. But it is worthy of note that the first recorded instance of Perpetual Adoration antedates Corpus Christi, and occurred at Avignon. On 11 September 1226, in compliance with the wish of Louis VII, who had just been victorious over the Albigensians, the Blessed Sacrament, veiled, was exposed in the Chapel of the Holy Cross, as an act of thanksgiving. So great was the throng of adorers that the bishop, Pierre de Corbie, judged it expedient to continue the adoration by night, as well as by day, a proposal that was subsquently ratified by the approval of the Holy See.
It is interesting to note the propagation in France during the seventeenth and eighteenth centuries of the Perpetual Adoration in all the churches and chapels of certain dioceses. The earliest mention of this practices is in 1658, when the churches in the Diocese of Chartres were opened for this purpose from six o'clock in the morning to six in the evening and wherever there were religious communities possessing a chapel the adoration was continued day and night. - Perpetual Adoration