The vision of Pope Leo XIII

The Prayer to St. Michael was composed by Pope Leo XIII after hearing two voices in a vision. One of Jesus and one of Satan on October 13, 1884. This is what he heard:

The guttural voice, the voice of Satan in his pride, boasted to Our Lord: "I can destroy your Church."
The gentle voice of Our Lord: "You can? Then go ahead and do so."
Satan: "To do so, I need more time and more power."
Our Lord: "How much time? How much power?
Satan: "75 to 100 years, and a greater power over those who will give themselves over to my service."
Our Lord: "You have the time, you will have the power. Do with them what you will."

This is the Prayer to St. Michael which was to be prayed at the end of every "low" Mass throughout the universal Church, along with the Salve Regina (Hail, Holy Queen). The practice of the congregation praying these prayers at the end of Mass continued until about 1970, with the introduction of the new rite of the Mass. Most catholics today , pray it before or after the rosary however:

"Saint Michael, the Archangel, defend us in battle; be our protection against the wickedness and snares of the devil. May God rebuke him, we humbly pray, and do thou, O prince of the heavenly host, by the power of God, thrust into Hell, Satan and all the other evil spirits, who prowl throughout the world, seeking the ruin of souls. Amen."

Pope John Paul II urged Catholics to recite this prayer . In his Angelus message given in St. Peter’s Square, Sunday, April 24, 1994 he said :

"The prayer can fortify us for that spiritual battle about which the Letter to the Ephesians speaks [of]: "Finally, draw your strength from the Lord and from his mighty power."(Ephesians 6:10). And to this same battle that the Book of the Apocalypse refers [to], recalling in front of our eyes the image of St. Michael the Archangel (cf. Revelations 12:7). Surely, this scene was very present to Pope Leo XIII, when, at the end of the previous century, he introduced to the entire Church a special prayer to St. Michael: ‘St. Michael the Archangel, defend us in battle; be our protection against the wickedness and snares of the devil… ’
"Even if today this prayer is no longer recited at the end of the Eucharistic celebration, I invite all to not forget it, but to recite it in order to obtain help in the battle against the forces of darkness and the spirit of this world."

[ taken from the article in the Michael Journal under church teachings - The vision of Pope Leo XIII, the Prayer to St. Michael]