Shekalim, Chapter 6, Mishnah 1. This mishnah lists a series of things in the Beis HaMikdash that came in sets of thirteen.
Thirteen in the Mikdash:
"Shloshah asar shofaros" - thirteen shofaros. As mentioned earlier in the tractate, the term 'shofar' refers to a charity box shaped like a shofar: the coin was inserted at the narrow, open end, and one could not reach a hand inside because of its narrowness, while it was opened from below, at the wide end. The mishnah further on will explain the purpose of the thirteen boxes and what each of the collections was designated for.
"Shloshah asar shulchanos" - thirteen tables. There were thirteen different tables in the Beis HaMikdash.
"Shalosh esrei hishtachava'os" - thirteen prostrations. There were thirteen places in the Beis HaMikdash where a person would bow down as he passed them.
The mishnah continues and relates that "shel beis Rabban Gamliel v'shel beis Rabbi Chanina segan haKohanim hayu mishtachavim arba esrei" - the members of the household of Rabban Gamliel and the members of the household of Rabbi Chanina, the deputy Kohen Gadol, would bow down fourteen times, since they had an additional place of prostration.
What the thirteen prostrations corresponded to, according to the various opinions, we will see further on. But where was the additional, fourteenth prostration that these families added?
Regarding this the mishnah says that it was "k'neged dir ha'eitzim" - opposite the wood chamber, the room where the wood designated for the altar was stored and where it was inspected.
"Shekein masores b'yadam me'avoseihem sheSham ha'aron nignaz" - they had a tradition from their forefathers that this was the place where King Yoshiyahu hid away the Aron in the days of the First Temple. And because of the sanctity of the Aron, even though it was buried deep beneath that spot, they would bow down there.
In summary: This mishnah enumerated thirteen shofaros (the charity boxes), thirteen tables and thirteen prostrations. The household of Rabban Gamliel and the household of Rabbi Chanina, the deputy Kohen Gadol, added a fourteenth prostration opposite the wood chamber, based on the tradition of their forefathers that in that place King Yoshiyahu hid away the Aron.
Further on we will deal in detail with the purpose of the thirteen boxes and with the explanation of the locations of the thirteen prostrations according to the various opinions.