Midrasch zu Schemot 31:11

8) R. Yossi says: I read "korban," and I might think an individual may donate (even) incense, and that he is bound by (Devarim 23:24): "What issues from your lips shall you observe and you shall do, and that he satisfies (Shemoth 30:9) "You shall not bring upon it (the altar) strange (i.e., donated) incense, nor burnt-offering" by not bringing it upon the inner altar, but he may bring it upon the outer altar (just as he does a burnt-offering) — it is, therefore, written (Shemoth 31:11): "and the spice incense for (smoking in the sanctuary, which is) holy" — spices are offered only within (i.e., in the sanctuary).