What can be said when applying ashes during Ash Wednesday Mass?

In our present liturgy for Ash Wednesday, we use ashes made from the burned palm branches distributed on the Palm Sunday of the previous year. The priest blesses the ashes and imposes them on the foreheads of the faithful, making the sign of the cross and saying, “Remember, man you are dust and to dust you shall return,” or “Turn away from sin and be faithful to the Gospel.”

The Ambrosian Rite (the liturgical rite from which Pope St. Paul VI was part of before being elected Supreme Pontiff) still to this day has no Ash Wednesday; it is therefore Quinquagesima that forms the prelude to Lent.

The Mozarabic Rite was the first to use ashes within the liturgical celebrations of the Church. Ashes were used prior to the Mozarabic Rite, but this was done outside of liturgical events, e.g., marking people for penance. - Mozarabic Rite

Memento, h*m*, quia pulvis es, et in pulverem reverteris (Remember that you are dust, and to dust you shall return) is obviously the most tradition liturgical formula used in the Roman Rite. Ashes were first introduce into the Roman Rite in 8th century but outside the Mass.

In the Middle Ages (at least by the time of the eighth century), those who were about to die were laid on the ground on top of sackcloth sprinkled with ashes. The priest would bless the dying person with holy water, saying, “Remember that thou art dust and to dust thou shalt return.” After the sprinkling, the priest asked, “Art thou content with sackcloth and ashes in testimony of thy penance before the Lord in the day of judgment?” To which the dying person replied, “I am content.” In all of these examples, the symbolism of mourning, mortality, and penance is clear.

Eventually, the use of ashes was adapted to mark the beginning of Lent, the 40-day preparation period (not including Sundays) for Easter. The ritual for the “Day of Ashes” is found in the earliest editions of the Gregorian Sacramentary which dates at least to the 8th century. About the year 1000, an Anglo-Saxon priest named Aelfric preached, “We read in the books both in the Old Law and in the New that the men who repented of their sins bestrewed themselves with ashes and clothed their bodies with sackcloth. Now let us do this little at the beginning of our Lent that we strew ashes upon our heads to signify that we ought to repent of our sins during the Lenten fast.” As an aside, Aelfric reinforced his point by then telling of a man who refused to go to Church on Ash Wednesday and receive ashes; the man was killed a few days later in a boar hunt. Since the Middle Ages, the Church has used ashes to mark the beginning of the penitential season of Lent, when we remember our mortality and mourn for our sins. - What are the origins of Ash Wednesday and the use of ashes?

Dom Gueranger has this to say about the origins of the Church's usage on the imposition of ashes on Ash Wednesday:

The recollection of what we are and what we are to be, would have checked that haughty rebellion, which has so often led us to break the law of God. And if, for the time to come, we would preserve in loyalty to Him, we must humble ourselves, accept the sentence, and look on this present life as a path to the grave. The path may be long or short; but to the tomb it must lead us. Remembering this, we shall see all things in their true light. We shall love that God, who has deigned to set His heart on us notwithstanding our being creatures of death: we shall hate, with deepest contrition, the insolence and ingratitude, wherewith we have spent so many of our few days of life, that is, in sinning against our heavenly Father: and we shall be not only willing, but eager, to go through these days of penance, which He so mercifully gives us for making reparation to His offended justice.

This was the motive the Church had in enriching her liturgy with the solemn rite, at which we are to assist this morning. When, upwards a thousand years ago, she decreed the anticipation of the Lenten fast by the last four days of Quinquagesima week, she instituted this impressive ceremony of signing the forehead of her children with ashes, while saying to them those awful words, wherewith God sentenced us to death: 'Remember, O man, that thou art dust, and into dust thou shalt return!' But the making use of ashes as a symbol of humiliation and penance, is of a much earlier date than the institution to which we allude. We find frequent mention of it in the Old Testament. Job, though a Gentile, sprinkled his flesh with ashes, that, thus humbled, he might propitiate the divine mercy and this was two thousand years before the coming of our Savior. The royal prophet tells us of himself, that he mingled ashes with his bread, because of the divine anger and indignation. Many such examples are to be met with in the sacred Scriptures; but so obvious is the analogy between the sinner who thus signifies his grief, and the object whereby he signifies it, that we read such instances without surprise. - Dom Prosper Gueranger on Ash Wednesday

Paenitemini, et credite Evangelion (Repent, and believe in the Gospel) was originally introduced as a liturgical option in the Mass of Pope Paul VI in 1969.

The website liturgies.net appears to have a full copy of the Roman Missal, including the propers of the Mass for Ash Wednesday. This states:

[The priest] sprinkles the ashes with holy water, without saying anything.

Then the Priest places ashes on the head of all those present who come to him, and says to each one:

Paenitemini, et credite Evangelion.
Repent, and believe in the Gospel.

Or:

Memento, h*m*, quia pulvis es, et in pulverem reverteris.
Remember that you are dust, and to dust you shall return.

I am attempting to find where in the Missal the statement originates.