Viet Dietrich (also known as Vitus Theodorus), the Nuremberg Reformer who was Martin Luther 's close friend, and who served as amanuensis both to Luther and Philipp Melanchthon (1506-1549), wrote a letter to Melanchthon dated June 30, 1530, which states:
Nullus abit dies, quin ut minimum tres horas, easque studiis aptissimas in orationibus ponat.
[Luther], during his retirement in the castle at Coburga for the safety of his person, having then more time to spare for devotion than his many public employments had been wont to afford him, was no niggard of it: but (as one Vitus Theodorus, who then lived with him, informed Melancthon,) spent no less in prayer to God, then at least three hours every day, and those such hours as were fittest for study.*
* Here Arrowsmith cites the Latin text of Dietrich's June 30, 1530 letter to Melanchthon.
The quote widely attributed to Luther, it appears to me, is likely an extrapolation from this reference by Viet Dietrich in his letter to Philipp Melanchthon. While Luther may not have uttered the words repeated so often today, yet they do, it seems, accurately reflect the importance of prayer in his life, and his own specific practice to pray three hours a day, early in the day, at least while he resided at the Veste Coburg.