וְאִ֨ם עַֽל־הַמִּשְׁכָּ֜ב ה֗וּא א֧וֹ עַֽל־הַכְּלִ֛י אֲשֶׁר־הִ֥וא יֹשֶֽׁבֶת־עָלָ֖יו בְּנָגְעוֹ־ב֑וֹ יִטְמָ֖א עַד־הָעָֽרֶב׃
And if he be on the bed, or on any thing whereon she sitteth, when he toucheth it, he shall be unclean until the even.
Read kabbalistic (mystical) interpretation of Leviticus 15:23: and classical mystical sources.