משנה על במדבר 19:22

If a [bed] leg that had contracted midras uncleanness was joined to a bed, all the bed contracts midras uncleanness. If it was subsequently taken off, it retains its midras uncleanness while the bed is unclean from contact with midras. If a bed leg that was subject to a seven-day uncleanness was joined to a bed, all the bed contracts seven-day uncleanness. If it was subsequently taken off it remains subject to seven-day uncleanness while the bed is only subject to evening-uncleanness. If a leg that was subject to evening uncleanness was joined to a bed, all the bed contracts evening uncleanness. If it was subsequently taken off it is still subject to evening uncleanness while the bed becomes clean. The same law applies also to the prong of a mattock.