Tractate Kelim - Chapter 19 - Mishnah 5 כלים פרק י״ט משנה ה׳

If around a bed that had contracted midras uncleanness one wrapped a mattress, the whole becomes subject to midras uncleanness. If it was removed, the bed remains subject to midras uncleanness but the mattress is unclean only from contact with midras. If around a bed that had contracted seven-day uncleanness one wrapped a mattress, the whole becomes subject to seven-day uncleanness. If it was removed, the bed remains subject to seven-day uncleanness but the mattress is unclean until the evening. If the bed was subject to evening uncleanness and around it he wrapped a mattress, the whole becomes subject to evening uncleanness; If it was removed, the bed remains subject to evening uncleanness but the mattress becomes clean.