Sefer HaBahir

[Mar Rachumai continued,] "One text says, 'And all the people saw the voices' (Exodus 20:15), and another text says, 'You heard the voice of words' (Deuteronomy 4:12). How can this be? At first, 'they saw the voices.' What did they see? Seven voices about which David spoke. And at the end, they heard the speech that comes out from between them all. But have we not learned that [that they are] ten? The rabbis said that all of them [the commandments] were spoken in one utterance (BT Shevuot 20b). So too, all of them were spoken in one utterance. But there were seven utterances! There were seven voices, and regarding three of them, it is said, 'You heard the voice of words, but you did not see an image — only a voice' (Deuteronomy 4:12). Thus, you learned that all of them were spoken as one utterance, so that Israel would not err by saying that another had helped him or the angels [helped] as His voice alone could not be this strong. Because of this, He returned and included all of them."