In the first Mishna in Massekhet Shekalim (2a) we are taught that on the first day of the month of Adar we announce that people should begin to bring their shekalim. The Mishna teaches a number of other activities that take place during Adar, among them the celebration of Purim, and public works that need to be done as the rainy season in Israel draws to a close. These public works projects include a number of activities in preparation for the groups of people who will be traveling to Jerusalem for Pesah – for example, clearing the roads and mikva’ot and marking graves so that the people who are coming to bring sacrifices will not, inadvertently, become ritually defiled by contact with a grave and be unable to enter the Temple.
The Gemara asks: From where is the obligation of marking graves derived?…Rabbi Ila in the name of Rabbi Shmuel bar Nahman cited a different verse in this regard: “And when they that pass through shall pass through the land, and anyone sees a man’s bone, then shall he set up a sign by it, till the buriers have buried it in the valley of Hamon-gog” (Yehezkel 39:15). This verse explicitly states that there is a need to mark graves.
This passage from Yehezkel describes the calamity of the war of Gog and Magog, and how it will take seven months for all of the dead to be properly buried so that the land of Israel will once again be tahor (ritually pure). The prophet describes the method that is to be used to carefully mark the graves, bone by bone.
This source for the halakha that graves must be marked (see Rambam, Hilkhot Tum’at Met 8:9) appears in Massekhet Mo’ed Katan, while it is introduced as a remez – a hint – to the law, rather than as the actual source. Given the clarity of the story in Yehezkel, many of the commentaries ask why the passage is only considered a remez.
From Rashi it appears that since it is not presented as an obligation, but rather as a story, it cannot be considered a true source.
Tosafot suggest that the story can only be considered a hint to the halakha because it is a description of an event that will take place “at the end of days.” Such a story cannot be the source for a present day halakhic obligation.
It should be noted that our Gemara, which as we explained above is Yerushalmi, presents this as a true source text, not simply as a remez. In fact, it is not uncommon to find the Bavli discounting a source unless it appears in the hamisha humshei Torah (the Five Books of Moses), while the Yerushalmi accepts other sources from Tanakh as well.