This Roman Catholic prayer is variously attributed to Saint Bernard of Clairvaux

Bernard of Clairvaux

Bernard of Clairvaux, O.Cist was a French abbot and the primary builder of the reforming Cistercian order.After the death of his mother, Bernard sought admission into the Cistercian order. Three years later, he was sent to found a new abbey at an isolated clearing in a glen known as the Val...

 or to St. Gertrude or St. Mechtilde.

In English:

'"O Loving Jesus, Meek Lamb of God, I, a miserable sinner, salute and worship the most Sacred Wound of Thy Shoulder on which Thou didst bear Thy heavy Cross, which so tore Thy Flesh and laid bare Thy Bones as to inflict on Thee an anguish greater than any other wound of Thy Most Blessed Body. I adore Thee, O Jesus most sorrowful; I praise and glorify Thee and give Thee thanks for this most sacred and painful Wound, beseeching Thee by that exceeding pain and by the crushing burden of Thy heavy Cross, to be merciful to me, a sinner, to forgive me all my mortal and venial sins and to lead me on towards Heaven along the Way of Thy Cross. Amen."

According to St. Bernard, he asked Jesus

Jesus

Jesus of Nazareth , commonly referred to as Jesus Christ or simply as Jesus or Christ, is the central figure of Christianity...

 which was His greatest unrecorded suffering and the wound that inflicted the most pain on Him in Calvary

Calvary

Calvary or Golgotha was the site, outside of ancient Jerusalem’s early first century walls, at which the crucifixion of Jesus is said to have occurred. Calvary and Golgotha are the English names for the site used in Western Christianity...

 and Jesus answered:

"I had on My Shoulder, while I bore My Cross on the Way of Sorrows, a grievous Wound which was more painful than the others and which is not recorded by men. Honor this Wound with thy devotion and I will grant thee whatsoever thou dost ask through its virtue and merit and in regard to all those who shall venerate this Wound, I will remit to them all their venial sins and will no longer remember their mortal sins."

In the twelfth century Pope Eugenius III approved of the promises with regards to this prayer . The modern version of the prayer bears the imprimatur

Imprimatur

An imprimatur is, in the proper sense, a declaration authorizing publication of a book. The term is also applied loosely to any mark of approval or endorsement.-Catholic Church:...

of Bishop Thomas D. Bevan .

The Prayer in Latin

O Iesu amantissime, Agne Dei mansuetissime, ego miser peccator saluto et veneror sacratissimum vulnus, quod in humero tuo, dum gravem crucis tuae trabem portares, persensisti : ob quod singularem dolorem et cruciatum in benedicto corpore tuo sustinuisti. Adoro te, Iesu afflictissime, et ex intimo corde laudo, benedico et glorifico te gratiasque ago pro hoc sacratissimo poenosissimoque vulnere humeri tui, humiliter deprecans, ut ob nimium illum dolorem, quem illud tibi inflixit, et propter grave onus crucis tuae, quod te tam dire afflixit, miserearis mihi peccatori, peccata mea venialia et mortalia remittas meque per viam crucis tuae ad caelum deducas. Amen.