US-P18.100ab ब्रह्मा दाशरथेर्यद्वदुक्त्यैवापानुदत् तमः ।
US-P18.100cd तस्य विष्णुत्वसंबोधे न यत्नान्तरमूचिवान् ॥
99, 100. Just as Brahmâ removed the Ignorance' of the son of Daśaratha by means of words only, but did not teach him any action in order that he might know that he was Vishnu; so, in order that one's ignorance may be removed the Śruti, teaches one 'Thou art That' when one has learnt the meanings of the subordinate sentences according to the Śrutis and popular word-meanings.
The statement of Brahmā addressed to Rāma is there in the Valmiki Ramayana.
Sanskrit
The Ancient Advaitic text Sankshepa shariraka of Sarvajnatma Muni too has a verse on the Rama-Brahma episode.
The Upadeśa Sāhasrī is cited multiple times in the Naiṣkarmya Siddhi. The author of the Naiṣkarmya Siddhi, Sureśvarācārya (a direct disciple of Adi Shankara), explicitly quotes verses from the Upadeśa Sāhasrī to validate his own teachings and establish authoritative support for his arguments from a recognized source.
warm regards
subbu