Māṇdūkya Upaniṣad 2.1.8

The seven senses (prana) also spring from him, the seven lights (acts of sensation), the seven kinds of fuel (objects by which the senses are lighted), the seven sacrifices (results of sensation), these seven worlds (the places of the senses, the worlds determined by the senses) in which the senses move, which rest in the cave (of the heart), and are placed there seven and seven.