Quotation sur Shabbat 2:12

For, as long as Sarah was alive a candle burned... They correspond to the three mitzvos which women are commanded (Shabbos 2:6). Corresponding to challah, there was a constant blessing. Corresponding to Shabbos candles, a lamp burned. And the cloud which hung [over the tent] represented the Shechinah. It was there due to the holiness of purity: she carefully observed [the laws of] nidah and would purify herself. And purity brings ruach hakodesh (Avodah Zarah 20b). (Gur Aryeh)