In the fifth aliyah G-d tells Moshe to hew new tablets - luchos - from a sapphire quarry that was under Moshe's tent. G-d says that He will write the words that were on the first luchos on this second set of luchos.
Moshe goes up Mt. Sinai early in the morning, and G-d descends in a cloud, and Moshe proclaims the thirteen attributes of mercy.
Moshe said: "G-D - G-D", meaning merciful before the sin as well as after the person repents. "Erech apayim" - "long suffering", meaning slow to anger; G-d waits - perhaps the person will repent. Merciful to the 2000th generation, repaying iniquity to the fourth generation (Rashi points out that the attribute of mercy is 500 times greater than the attribute of punishment). This passage is recited daily in the confessional prayer tachanun (siddur p. 62) when we are asking for mercy for our sins, and also many times during Ne-ila prayer at the close of Yom Kippur.