The query submitted is rather imprecise as I know no single 'prayer for the dead'. So I will draft a reply and you can take it from there.

Rite of Funerals

I am not sure what the questioner means. There are a number of prayers for the dead in regular use, meeting the various pastoral and human situations that may arise.

In the introduction to the Roman Ritual : Rite of Funerals, the following passage sets the background of the Church's Pastoral Care of the dying, and finally those who are dead. There is frequent references made to the needs of the relatives and friends of the deceased at the time of bereavement.

"In the funeral rites the Church celebrates the paschal mystery of Christ. Those who in baptism have become one with the dead and risen Christ will pass with him from death to life, to be purified in soul and welcomed into the fellowship of the saints in heaven. They look forward in blessed hope to the second coming and the bodily resurrection of the dead. 
The Church therefore celebrates the eucharistic sacrifice of Christ's passover for the dead, and offers prayers and petitions for them. In the communion of all Christ's members, the prayers which bring spiritual help to some may bring to others a consoling hope."

There are various rituals, and in different countries and cultures varying local customs. But broadly the Church has prepared a selection of texts and liturgical services for use in the home of the deceased, in the church, and at the site of burial or cremation. These consist in a generous selection of prayers, readings from scripture and other material suitable for the various locations, and depending on the pastoral needs and wishes of the relatives of the deceased.

Most commonly, at some point a Requiem Mass will be celebrated by the relatives and friends of the deceased in the local community. Frequently, this may take the form of a joyful celebration of the Resurrection, and expresses the hope of eternal life, and of redemption in Christ. In this sense, I would answer the question and say that this is the 'prayer for the dead.'

Final Commendation and Farewell
It is possible that your questioner has in mind the solemn final farewell.
The Latin 'edition typica' text is translated by local Episcopal Conferences and the English text, which is copyright, begins as follows:

"With faith in Jesus Christ,
we reverently bring the body of our brother (sister)
to be buried in its human imperfection.
Let us pray with confidence to God,
who gives life to all things,
that he will raise up this mortal body
to the perfection and the company of the saints.
May God give him (her) a merciful judgment
and forgive all his (her) sins.
May Christ, the Good Shepherd,
lead him (her) safely home
to be at peace with God our Father.
And may he (she) be happy for ever
with all the saints
in the presence of the eternal King,"

Private Prayers
A number of pious customs and prayers are in use in various countries and localties. It is possible that one such is known to the questioner as 'the prayer for the dead.' They are too many to describe, and no one may be identified with the question.

Scripture
There is a wide selection of passages from scripture which are the basis of Christian faith in the Resurrection of Jesus. Indeed the Letters of the Apostle Paul are a rich source, given that the preaching of the Apostle is founded on his encounter , nad that of the other Apostles, with the risen Lord. 

Perhaps the earliest scriptural reference to belief in eternal life is to be found in the Old Testament text:

Prophet Daniel 12 : 1 - 3
This text is remarkable as it is one of the earliest and clearest expressions of the belief in the resurrection of the dead and eternal life. 
This era at the end of time will be preceded by ³a time of great distress,² but the prophet encourages the people, ³all those whose names are found written in the Book...many will awake...to everlasting life.² 
Another Old Testament text worth reference is 2 Maccabees 12 :39 -45
Where Judas the Maccabean leader has sacrifices offered for those who had fallen in battle, "an altogther fine and noble action, in which he took full account of the resurrection. For if he had not expected the fallen to rise again it would have been superfluous and foolish to pray for the dead."

Peregrinator Ignotus