λανθάνειν τι ποιῶν
From LSJ
πάντα χωρεῖ καὶ οὐδὲν μένει καὶ δὶς ἐς τὸν αὐτὸν ποταμὸν οὐκ ἂν ἐμβαίης → all things move and nothing remains still, and you cannot step twice into the same stream
Greek > English (Woodhouse)
do a thing insensibly, do a thing unawares, do a thing undetected