At that time, God said to me, 'Carve out two stone tablets like the first ones, and come up to Me on the mountain. Make yourself a wooden ark.
Ba'et hahi amar Adonay elay psol-lecha shney-luchot avanim karishonim va'aleh elay haharah ve'asita lecha aron ets.
Exodus 34:1. This was on 1 Elul, after Moses had prayed for the people forty days (Rashi).
Some say that this was a temporary ark, that was used until the golden ark mentioned in Exodus 25:10 was made (Ramban; Bachya). According to others, this ark was used for the broken tablets, and it was taken out in war (Rashi; see Numbers 10:33). Some say that Moses made the basic ark out of acacia wood, and then Betzalel covered it with gold (Malbim; cf. Ibn Ezra; Yoma 3b, 72b).