What is the earliest instance of Eucharistic adoration?

Eucharistic adoration is a practice in Catholicism in which the Eucharist, being bread that has literally turned into the body of Christ, is publicly displayed. Commonly, it is placed in a monstrance and the faithful sit in silence and pray or otherwise reflect on the fact that they are in the presence of Christ's body. It is not consumed as it would be during Communion, and the faithful simply look at it. What is the earliest recorded instance of this practice? To be clear, I'm not talking about simply saving the Eucharist for the sick or a later time, but rather to have it displayed for adoration without consuming it.