This Prayer for the Dead (sometimes titled A Prayer for the Deceased) is traditionally attributed to Saint Ignatius of Antioch. Ignatius, the third bishop of Antioch in Syria (Saint Peter was the first bishop) and a disciple of Saint John the Evangelist, was martyred in the Colosseum in Rome by being fed to wild beasts. On his way to Rome from Syria, Saint Ignatius witnessed to the Gospel of Christ in preaching, epistles to Christian communities (including a famous Letter to the Romans and one to Saint Polycarp, bishop of Smyrna and the last of the disciples of the Apostles to meet his death by martyrdom), and the composing of prayers, of which this is reputed to be one.
Even if this prayer is of slightly later vintage and merely ascribed to Saint Ignatius, it still shows that Christian prayer for the dead, which implies belief in what would later come to be known as Purgatory, is a very early practice. This is a very beautiful prayer to pray during November, the month of the Holy Souls in Purgatory (and especially on All Souls Day), or at any time that you fulfill the Christian duty of praying for the dead.
Prayer for the Dead By Saint Ignatius of Antioch
Receive in tranquility and peace,
O Lord, the souls of your servants
who have departed this present life to come to you.
Grant them rest and place them
in the habitations of light,
the abodes of blessed spirits.
Give them the life that will not age,
good things that will not pass away,
delights that have no end,
through Jesus Christ our Lord.