Mishnah Shabbat
Shabbat 2:6-7
The Mishnah lists which wicks and oils are forbidden for Shabbat lamps due to the risk that one might tilt the lamp to improve the flame, and enumerates which materials are permitted. It also teaches that one must light the Shabbat candles before nightfall.
עַל שָׁלשׁ עֲבֵרוֹת נָשִׁים מֵתוֹת בִּשְׁעַת לֵדָתָן, עַל שֶׁאֵינָן זְהִירוֹת בַּנִּדָּה וּבַחַלָּה וּבְהַדְלָקַת הַנֵּר: שְׁלשָׁה דְבָרִים צָרִיךְ אָדָם לוֹמַר בְּתוֹךְ בֵּיתוֹ עֶרֶב שַׁבָּת עִם חֲשֵׁכָה. עִשַּׂרְתֶּם. עֵרַבְתֶּם. הַדְלִיקוּ אֶת הַנֵּר. סָפֵק חֲשֵׁכָה סָפֵק אֵין חֲשֵׁכָה, אֵין מְעַשְּׂרִין אֶת הַוַּדַּאי, וְאֵין מַטְבִּילִין אֶת הַכֵּלִים, וְאֵין מַדְלִיקִין אֶת הַנֵּרוֹת, אֲבָל מְעַשְּׂרִין אֶת הַדְּמַאי, וּמְעָרְבִין, וְטוֹמְנִין אֶת הַחַמִּין:
This mishna concludes the aggadic treatment of the topic of kindling the Shabbat lights. For three transgressions women are punished and die during childbirth: For the fact that they are not careful in observing the laws of a menstruating woman, and in separating ḥalla from the dough, and in lighting the Shabbat lamp. There are three things a person must say in his home on Shabbat eve at nightfall and not before. The mishna elaborates: He should ask the members of his household, have you tithed the crop that required tithing? Have you placed the eiruv for joining the courtyards and joining the Shabbat borders? If you have done so, light the lamp in honor of Shabbat. The Sages stated a principle: If the time arrives on Friday when there is uncertainty whether it is nightfall and uncertainty whether it is not yet nightfall, one may not tithe the crop that has definitely not been tithed, and one may not immerse ritually impure vessels in a ritual bath to render them ritually pure, and one may not light the Shabbat lights. However, one may tithe demai, doubtfully tithed produce, which must be tithed due to mere suspicion. And one may place an eiruv and insulate the hot water to be used on Shabbat.
Why it matters — The primary Mishnaic source defining the proper materials for Shabbat candles and the obligation to light before Shabbat.