Rashi on Numbers
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וידבר. במדבר סיני באחד לחדש וגו'. מִתּוֹךְ חִבָּתָן לְפָנָיו מוֹנֶה אוֹתָם כָּל שָׁעָה — כְּשֶׁיָּצְאוּ מִמִּצְרַיִם מְנָאָן, וּכְשֶׁנָּפְלוּ בָּעֵגֶל מְנָאָן לֵידַע מִנְיַן הַנּוֹתָרִים, כְּשֶׁבָּא לְהַשְׁרוֹת שְׁכִינָתוֹ עֲלֵיהֶם מְנָאָם, בְּאֶחָד בְּנִיסָן הוּקַם הַמִּשְׁכָּן וּבְאֶחָד בְּאִיָּר מְנָאָם:

וידבר… במדבר סיני … באחד לחדש וגו׳ AND [THE LORD] SPOKE [UNTO MOSES] IN THE DESERT OF SINAI … ON THE FIRST DAY OF THE [SECOND] MONTH … [TAKE YE THE SUM OF ALL THE CONGREGATION] etc. — Because they were dear to him, He counts them every now and then: when they went forth from Egypt He counted them (Exodus 12:37), when many of them fell in consequence of their having worshipped the golden calf He counted them to ascertain the number of those left (cf. Rashi Exodus 30:16); when he was about to make His Shechinah dwell amongst them (i. e. when He commanded them to make a Tabernacle), He again took their census; for on the first day of Nisan the Tabernacle was erected (Exodus 40:2) and shortly afterwards, on the first day of Iyar, He counted them.