Midrasz do Izajasza 48:3

"upon your hand": This is the left hand. You say this, but perhaps it is the right? Though there is no proof for this, it is intimated in (Isaiah 48:3) "My hand has also founded on earth, and My right hand has spanned the heavens," and (Judges 5:26) "Her hand reached for the tent pin, her right for the workmen's hammer," indicating that "hand," unqualified, is the left hand. R. Yonathan says (Devarim 6:8-9) "and you shall tie them … and you shall write them": Just as the writing (of the mezuzah) is with the right hand, so, the tying of the tefillin (on the left hand). Abba Yossi says: We find the right to be called "hand," and even though there is no proof (for its application here), it is intimated in (Genesis 48:17) "that his father placed his right hand." And what is the intent of "upon your hand"? To include an amputee (of the left hand), that his tefillin are placed on his right arm.