Shatapatha Brahmana 6.3.1.31

6.3.1.31

sā vaiṇavī syāt | agnirdevebhya udakrāmatsa veṇum prāviśattasmātsa suṣiraḥ sa etāni varmāṇyabhito 'kuruta parvāṇyananuprajñānāya yatra-yatra nidadāha tāni kalmāṣāṇyabhavan

It should be made of bamboo. Agni went away from the gods. He entered into a bamboo-stem; whence that is hollow. On both sides he made himself those fences, the knots, so as not to be found out; and wherever he burnt through, those spots came to be.