משנה על כלאים 3:3

If a mattress was wrapped around a bed and a corpse touched them, they are subject to a seven-day uncleanness; If they are taken apart they are still subject to a seven-day uncleanness. If a sheretz touched them they are subject to an evening uncleanness; If they are taken apart they are still subject to evening uncleanness. A bed from which the two longer sides were removed and two new ones were prepared for it but the original sockets were not changed: If the new sides were broken the bed retains its uncleanness, But if the old ones were broken it becomes clean, since all depends on the old ones.