レビ記 15:5

ואיש אשר יגע במשכבו AND WHOSOEVER TOUCHES HIS PLACE OF REPOSE [SHALL WASH HIS GARMENTS … AND BE UNCLEAN UNTIL THE EVENING] — This tells us regarding the object on which he lies down that it is more stringent than an object touched by him in any manner other than lying on it — for this (the former) becomes a primary source of uncleanness (אב הטומאה) communicating uncleanness to any man who touches it, he in turn making unclean the garments he happens to wear at that moment, while anything touched by him that is not his bed becomes only a secondary source of uncleanness of the first degree (ולד הטומאה) communicating uncleanness only to food and drink (but not to men or garments) (cf. Sifra, Metzora Parashat Zavim, Section 2 1).

וְאִ֕ישׁ אֲשֶׁ֥ר יִגַּ֖ע בְּמִשְׁכָּב֑וֹ יְכַבֵּ֧ס בְּגָדָ֛יו וְרָחַ֥ץ בַּמַּ֖יִם וְטָמֵ֥א עַד־הָעָֽרֶב׃

And whoever touches his bed shall wash his clothes, and bathe himself in water, and be unclean until evening.