וַתְּדַבֵּ֨ר מִרְיָ֤ם וְאַהֲרֹן֙ בְּמֹשֶׁ֔ה עַל־אֹד֛וֹת הָאִשָּׁ֥ה הַכֻּשִׁ֖ית אֲשֶׁ֣ר לָקָ֑ח כִּֽי־אִשָּׁ֥ה כֻשִׁ֖ית לָקָֽח׃
And Miriam and Aaron spoke against Moses because of the Cushite woman whom he had married; for he had married a Cushite woman.
Read kabbalistic (mystical) interpretation of Numbers 12:1: and classical mystical sources.