וּלְקַחְתֶּ֨ם לָכֶ֜ם בַּיּ֣וֹם הָרִאשׁ֗וֹן פְּרִ֨י עֵ֤ץ הָדָר֙ כַּפֹּ֣ת תְּמָרִ֔ים וַעֲנַ֥ף עֵץ־עָבֹ֖ת וְעַרְבֵי־נָ֑חַל וּשְׂמַחְתֶּ֗ם לִפְנֵ֛י יְהוָ֥ה אֱלֹהֵיכֶ֖ם שִׁבְעַ֥ת יָמִֽים׃
And ye shall take you on the first day the fruit of goodly trees, branches of palm-trees, and boughs of thick trees, and willows of the brook, and ye shall rejoice before the LORD your God seven days.
Tosefta Sukkah
The [beat of the willow] is a tradition from Moses at Sinai, and Abba Sha'ul deduced it from Scripture, as it is says, “Willows of the brook”, the plural denoting two, one for the lulav, and one for the altar. Rabbi Elieser ben Yacov said, Thus were they saying, "To Him and to thee, O altar, to Him and to thee, O altar!" Eighteen days and one night (in the year) the entire Hallel is repeated. These are: the eight days of sukkot, the eight days of Hanukkah, the first day of Passover, the night of the first day of Passover, and the first day of Shavuot.