Midrasch zu Bamidbar 18:13

8) And whence is it derived that they (the two loaves) precede the (bringing of) the first-fruits (bikkurim)? From (Shemoth 34:22): ("And the festival of Shavuoth shall you make for yourself,) the first of the wheat harvest" (i.e., What you make on Shavuoth [the two loaves] should precede [all that comes of] the wheat harvest.) This tells me (that it precedes only bikkurim) of wheat. Whence do I derive (that it precedes also bikkurim) of barley? From (Shemoth 23:16): "which you sow" (implying all that you sow.) This tells me only of what is sown. Whence do I derive (that the two loaves precede) what grows of itself (from seeds scattered by the wind)? From (Shemoth 23:16): "in the field" (implying all that grows in the field.) This tells me only of what grows in the field. Whence do I derive the same for (fruits planted on) a roof, in a yard, or in a ruin? From (Bamidbar 18:13): (The two loaves shall be) "the first-fruits of all that is in their land." And whence is it derived that they precede the (bikkurim of) libations and the fruits of the tree? From (Bamidbar 23:16): (The two loaves shall be) "the first fruits of your labor," and (Bamidbar 23:16): "when you gather your labor from the field." (Grapes for libations and fruits are subsumed in "gathered.")