The Duties of the Heart, by Rabbi Bachye, tr. by Edwin Collins, , at sacred-texts.com
But it is forbidden to us to study it for purposes of worldly advantage; but from the single motive of Love Alone. The Rabbis say: "Do things for the sake of the work itself, and speak the words [of the law] for their own sake. Thou shalt neither make the Torah a crown, to magnify thyself with it, nor a spade to dig with." And they say, in reference to words of Psalm cxii., "'Happy is the man who fears the Eternal, and delighteth greatly in His Commandments,' Rabbi Eliezar explains delighteth greatly in His Commandments, themselves, and not in the merit, or in any reward, spiritual or material, attached to their performance, as we have learned,' 'be not like servants that serve their master for the sake of receiving a reward, but be ye like servants who serve their master with the intention of receiving no reward, and let the awe of Heaven be upon you.'"