Rashi on Leviticus 15:4/רש"י על ויקרא ט״ו:ד׳

1כל המשכב EVERY PLACE OF REPOSE — This means: everything that is fitted for sleeping on (thus excluding a door or plank etc.). One might, however, think that it includes anything intended for some other action also, as long as it can be used for sleeping and he actually slept upon it! It, however, states אשר ישכב — it does not state אשר שכב, “upon which one has slept" but אשר ישכב, “upon which one sleeps usually" — i.e. that which is intended to be used for this always, thus excluding that of which people can say to him, “Get up and let us to our work [with that upon which you are lying]" (Sifra, Metzora Parashat Zavim, Chapter 1 1-3).

2אשר ישב [EVERYTHING] WHEREON HE SITTETH — It is not said here ‎יָשַׁב ‎‏ אשר, "on which he has sat", but אשר יֵשֵׁב, “on which one sits usually" — it speaks therefore of something that is intended to be used for this always (Sifra, Metzora Parashat Zavim, Chapter 1 3; Shabbat 59a).