Śatapatha Brāhmaṇa ("of a hundred paths") 13.4.4.9

13.4.4.9

asthibhya evāsya khadiraḥ samabhavat tasmātsa dāruṇo bahusāro dāruṇamiva hyasthi tenaivainaṃ tadrūpeṇa samardhayatyantare bailvā bhavanti bāhye khādirā antare hi majjāno bāhyānyasthīni sva evaināṃstadāyatane dadhāti

From his (Pragâpati's) bones the khadira, was produced, whence that (tree) is hard and of great strength, for hard, as it were, is bone: with that quality he thus endows it. The bilva (stakes) are inside, and the khadira ones outside, for inside is the marrow, and outside the bones: he thus puts them in their own place.