One of his quotes on the Holy Eucharist...
"Material food first changes into the one who eats it, and then, as a consequence, restores to him lost strength and increases his vitality. Spiritual food, on the other hand, changes the person who eats it into itself. Thus the effect proper to this Sacrament is the conversion of a man into Christ, so that he may no longer live, but Christ lives in him; consequently, it has the double effect of restoring the spiritual strength he had lost by his sins and defects, and of increasing the strength of his virtues."
At the request of Pope Urban IV, who extended the celebration of the Feast of Corpus Christi to the universal church, St. Thomas Aquinas composed the office (the official prayers of the Church) for the feast. This office is the source of the famous Eucharistic hymns "Pange Lingua Gloriosi" and "Tantum Ergo Sacramentum" (the final two verses of the "Pange Lingua").
Today, Catholics are familiar with the "Pange Lingua" primarily from its use during the procession at the Mass of the Lord's Supper on Holy Thursday evening, when the Body of Christ is removed from the tabernacle and transferred to another place to be kept overnight, while the altar is stripped bare.