Midrasch zu Dewarim 11:18

Our Rabbis were taught (Deut. 11, 18) Therefore shall ye lay (V'sam-tam) i.e., an unfailing remedy. The Torah is likened unto a medicine of life. This may be compared unto a man who beat his son and wounded him severely. He applied a plaster unto the wound and warned him, saying: "My son, as long as this plaster will be on your wound you may eat and drink whatever you please; you may even bathe yourself in warm or cold water and you need not be afraid; but as soon as you will remove it, you will cause an ulceration." So also has the Holy One, praised be He, said unto Israel: "I have created an evil inclination and I have also created the Torah with which to season (temper) it. If you will study the Torah then you will not be surrendered into his hand, as it is said (Gen. 4, 7) If thou dost well, then it shall he lifted up. But if you will not study the Torah then you will be surrendered into his hand, as it is said (Ib., ib., ib.) Sin coucheth at the door; and not only this, but he will spend all his energy to induce you to sin, as it is said in the previous passage, And unto thee is its desire. However, if you desire you can rule over it, as it is said (Ib., ib., ib.) But thou canst rule over it."

"upon your hand": upon the upper arm. But perhaps the hand (i.e., the palm) literally? It follows (that the upper arm is meant), viz.: The Torah prescribes tefillin for both the hand and the head. Just as the head (connotes) the height of the head, so, the hand (connotes) the height of the hand (i.e., the upper arm). R. Eliezer says: "upon your hand": upon the upper arm. You say upon the upper arm, but perhaps the hand (literally [i.e., the palm]) is meant? It is, therefore, written (Ibid.) "to you as a sign, and not to others as a sign" (i.e., the palm is generally exposed; the upper arm, not.) R. Yitzchak says: On the upper arm or on your hand (i.e., your palm), literally? It is, therefore, written (Devarim 11:18) "And you shall place these words upon your heart" — what is aligned with your heart, i.e., the upper arm.

And ten thousand at thy right hand. Why does it say a thousand at thy left hand, and ten thousand at thy right hand? The left hand does not require as many angels, because the name of the Holy One, blessed be He, is inscribed on the phylactery which is placed on the left hand, as it is said: Ye shall bind them for a sign upon thy hand (Deut. 11:18).