The Decade Prayer for the rosary, commonly known as the Fatima Prayer, was revealed by Our Lady of Fatima on July 13, 1917. She asked the three shepherd children, Francisco, Jacinta, and Lucia, to recite it at the end of each decade of the rosary. It was approved for public use in 1930, and has since become a common (though optional) part of the rosary.
The Fatima Prayer
O my Jesus, forgive us our sins, save us from the fires of hell, and lead all souls to Heaven, especially those in most need of Thy mercy.