This prayer from Anselm of Canterbury (c. 1033–1109) can be found in his famous Proslogion, which he wrote in 1077-1078.
“I pray, O God, that I may know You and love You, so that I may rejoice in You. And if I cannot do so fully in this life may I progress gradually until it comes to fullness. Let the knowledge of You grow in me here, and there [in heaven] be made complete; let Your love grow in me here and there be made complete, so that here my joy may be great in hope, and there be complete in reality. Lord, by Your Son You command, or rather, counsel us to ask and you promise that we shall receive so that our ‘joy may be complete’ [John 16:24]. I ask, Lord, as You counsel through our admirable counselor. May I receive what You promise through Your truth so that my ‘joy may be complete’. God of truth, I ask that I may receive so that my ‘joy may be complete’. Until then let my mind meditate on it, let my tongue speak of it, let my heart love it, let my mouth preach it. Let my soul hunger for it, let my flesh thirst for it, my whole being desire it, until I enter into the ‘joy of the Lord’ [Matt. 25:21], who is God, Three in One, ‘blessed forever. Amen’ [Rom 9:5].”