Anandakanda 1.15.45

1.15.45

<brahmavṛkṣamūlaguptadhātrīkalpaḥ>
atisthūlataraṃ dīrghaṃ bhedayedbrahmabhūruham /
mūle trihastaśeṣaṃ ca tadagre gartam āracet //

<Preparation of Āmalakī Concealed in the Root of the Brahmā Tree>
One should split a very thick and tall brahmavṛkṣa, / and leaving three hastas at the root, hollow out a cavity at its top.