וַיֹּ֤אמֶר יְהוָה֙ אֶל־מֹשֶׁ֔ה כֹּ֥ה תֹאמַ֖ר אֶל־בְּנֵ֣י יִשְׂרָאֵ֑ל אַתֶּ֣ם רְאִיתֶ֔ם כִּ֚י מִן־הַשָּׁמַ֔יִם דִּבַּ֖רְתִּי עִמָּכֶֽם׃
And the LORD said unto Moses: Thus thou shalt say unto the children of Israel: Ye yourselves have seen that I have talked with you from heaven.
Read kabbalistic (mystical) interpretation of Exodus 20:19: and classical mystical sources.