Moshe prayed for forty days on Sinai, as the Torah tells us –
וָאֶתְנַפַּל לִפְנֵי יְדֹוָד אֶת אַרְבָּעִים הַיּוֹם וְאֶת אַרְבָּעִים הַלַּיְלָה אֲשֶׁר הִתְנַפָּלְתִּי כִּי אָמַר יְדֹוָד לְהַשְׁמִיד אֶתְכֶם
And I fell in front of God in prayer for forty days and forty nights, because he wanted to destroy you
בָּעֵת הַהִוא אָמַר יְדֹוָד אֵלַי פְּסָל לְךָ שְׁנֵי לוּחֹת אֲבָנִים כָּרִאשֹׁנִים וַעֲלֵה אֵלַי הָהָרָה וְעָשִׂיתָ לְּךָ אֲרוֹן עֵץ
וְאֶכְתֹּב עַל הַלֻּחֹת אֶת הַדְּבָרִים אֲשֶׁר הָיוּ עַל הַלֻּחֹת הָרִאשֹׁנִים אֲשֶׁר שִׁבַּרְתָּ וְשַׂמְתָּם בָּאָרוֹן
At that time, the Lord said to me, "Hew for yourself two stone tablets like the first ones and come up to Me onto the mountain, and make for yourself a wooden ark, And I shall inscribe on the tablets the words that were upon the first tablets which you shattered and you shall place them into the ark.
Rashi explains that this happened on Rosh hodesh Elul, on his day he was commanded to climb Sinai again to receive the tablets, that this time the tablets will be made by Moshe, unlike the first set that were made by god. These tablets will be put in a wooden ark, something we were not commanded with the first set.