He wrote on the tablets the words of the covenant—the Ten Commandments

 Exodus 34-35

The Lord said to Moses, “Chisel out two stone tablets like the first ones, and I will write on them the words that were on the first tablets, which you broke.

Moses bowed to the ground at once and worshiped. “Lord,” he said, “if I have found favor in your eyes, then let the Lord go with us. Although this is a stiff-necked people, forgive our wickedness and our sin, and take us as your inheritance.”

Then the Lord said: “I am making a covenant with you. Before all your people I will do wonders never before done in any nation in all the world. The people you live among will see how awesome is the work that I, the Lord, will do for you. 

Be careful not to make a treaty with those who live in the land; for when they prostitute themselves to their gods and sacrifice to them, they will invite you and you will eat their sacrifices. And when you choose some of their daughters as wives for your sons and those daughters prostitute themselves to their gods, they will lead your sons to do the same.

Do not make any idols.  “Celebrate the Festival of Unleavened Bread. For seven days eat bread made without yeast, as I commanded you. Do this at the appointed time in the month of Aviv, for in that month you came out of Egypt.

Bring the best of the firstfruits of your soil to the house of the Lord your God.

Moses was there with the Lord forty days and forty nights without eating bread or drinking water. And he wrote on the tablets the words of the covenant—the Ten Commandments.

Then the whole Israelite community withdrew from Moses’ presence, and everyone who was willing and whose heart moved them came and brought an offering to the Lord for the work on the tent of meeting, for all its service, and for the sacred garments.

Why?

The Lord face to face talk to Moses about thecovenant- the Ten Commandments. This is let Israelite remember they belong to the Lord and they were chosen by the Lord, so they can not have any treaty or marriage with foreign people.

Even the Lord mentioned these Ten Commandments, but finally Israelite could not obey these commandments after a few years.

How?

All of us are weak and do not have enough faith to obey the word of the Lord, so we need to pray, let the Lord give me the strength, faith, and love to obey God's commandments.