Sefer Chasidim: Sing Your Prayers
If you set your prayers to a pleasant melody that you enjoy, you will pray with more devotion, and your feelings will be aroused by the words you utter. When you petition God for something, select a melody that puts you in a prayerful mood. When you sing His praises, choose a melody that evokes happiness, so that your mouth brims with love and joy for Him who knows your thoughts, and you will praise Him with deep ardor and jubilation. Sefer Chasidim #158 – Translated by Avraham Yaakov Finkel