It’s this one, from Jerome’s Letter 108, his panegyric to Paula at the time of her death:
Inflamed by their virtues she thought more and more each moment of forsaking her home. Disregarding her house, her children, her servants, her property, and in a word everything connected with the world, she was eager — alone and unaccompanied (if ever it could be said that she was so)— to go to the desert made famous by its Pauls and by its Antonies. And at last when the winter was over and the sea was open, and when the bishops were returning to their churches, she also sailed with them in her prayers and desires. Not to prolong the story, she went down to Portus accompanied by her brother, her kinsfolk and above all her own children eager by their demonstrations of affection to overcome their loving mother. At last the sails were set and the strokes of the rowers carried the vessel into the deep. On the shore the little Toxotius stretched forth his hands in entreaty, while Rufina, now grown up, with silent sobs besought her mother to wait till she should be married. But still Paula’s eyes were dry as she turned them heavenwards; and she overcame her love for her children by her love for God. She knew herself no more as a mother, that she might approve herself a handmaid of Christ. Yet her heart was rent within her, and she wrestled with her grief, as though she were being forcibly separated from parts of herself. The greatness of the affection she had to overcome made all admire her victory the more. Among the cruel hardships which attend prisoners of war in the hands of their enemies, there is none severer than the separation of parents from their children. Though it is against the laws of nature, she endured this trial with unabated faith; nay more she sought it with a joyful heart: and overcoming her love for her children by her greater love for God, she concentrated herself quietly upon Eustochium alone, the partner alike of her vows and of her voyage. Meantime the vessel ploughed onwards and all her fellow-passengers looked back to the shore. But she turned away her eyes that she might not see what she could not behold without agony. No mother, it must be confessed, ever loved her children so dearly. Before setting out she gave them all that she had, disinheriting herself upon earth that she might find an inheritance in heaven.
As J.N.D. Kelly notes in his biography of Jerome:
If the rhetoric seems overdone, it remains a fact that the break she was making with home and family cut far deeper than any sacrifice of his (Jerome’s.)