Our light is our ideas: it is universal Reason, it is the intelligible substance that contains them. The truths that we know, are nothign other than the relations that are between these ideas. For it is clear [visible] that the relation of equality that exists between two & two & four is an immutable and necessary truth. By virtue of holding that the modalities of your soul are essentially representative, you are saying that you are your own light, your own wisdom, your own interior teacher. You render the honor due to the power of God, if you recognize that you are not the cause of your light. But you do not render the honor due to his wisdom in holding that your modalities are essentially representative of the truth, in holding that they are really and formally the light that enlightens you. You attribute to yourself what pertains only to universal Reason, who instructs you, you Sir, & all of the intelligences that there are, who only see the truth because they contemplate the intelligible substance that contains the Reason by which they are made, and without which nothing is intelligible.
[Malebranche, Réponse aux VFI, Chapter VII, section II (OC 6:64).]
This argument is adapted from St. Augustine; Say that you are not your own light is perhaps Malebranche's favorite Augustinian phrase. Likewise, the mention of the 'interior Teacher' highlights the Augustinian origin of the argument, since Malebranche borrows it from the De Magistro. The above passage gives Malebranche's primary and perpetual complaint against Arnauld.
Malebranche may never have read the De Magistro through -- despite the fact that people are constantly appealing to his authority, Augustine's original works seem not to have been widely available in the seventeenth century. What were available were florilegia and manuals of various kinds that quoted Augustine extensively and arranged passages from Augustine's works under various topics. In this way, Augustine was everywhere, but in modified form. Henri Gouhier established that Malebranche often draws directly from a manual by Ambrosius Victor, a.k.a. André Martin (French, scroll down).