Shiva Purana
सर्वात्मत्वं तयोरेवं ब्रह्मेत्युपनिषत्सु च ॥ गीयते ब्रह्मशब्देन बृंहिधात्वर्थगोचरम् ॥ ३५ ॥
The word Brahman in the Upaniṣads signifies Ātman collectively. It can be traced to the root Bṛh to increase or grow.
english translation
sarvAtmatvaM tayorevaM brahmetyupaniSatsu ca ॥ gIyate brahmazabdena bRMhidhAtvarthagocaram ॥ 35 ॥
hk transliteration by Sanscript