The Spirit itself beareth witness with our spirit, that we are the children of God:
Berean Study Bible (BSB) - Romans 8:16 >The Spirit Himself testifies with our spirit that we are God’s children.
Textus Receptus (TR) - Romans 8:16 >Αὐτὸ τὸ πνεῦμα συμμαρτυρεῖ τῷ πνεύματι ἡμῶν, ὅτι ἐσμὲν τέκνα Θεοῦ·