In Parshat Mishpatim (Shmot 24:12-14) God asks Moshe to come up to Mount Sinai:

God then said to Moshe, “Come up to Me to the mountain and remain there. I will give you the tablets of stone, the Torah and the commandment, which I have written in order to teach them.” Moshe and Yehoshua, his attendant, set out, but only Moshe went up to the mountain of God. He (Moshe) said to the elders, “Wait here for us until we return to you, and Aharon and Chur will be with you. Whoever has a claim can approach them.”

The first time that Chur was mentioned in the Torah was in Shmot 17:11-12, during the fight against Amalek:

When Moshe raised his hand, B’nai Yisrael prevailed: But when he let his hand down to rest, Amalek prevailed. Moshe’s hands became heavy. They took a stone and placed it under him and he sat upon it. Aharon and Chur supported his hands, one of them on one side. His hands remained an expression of faith until sunset.

Who was Chur? How was he chosen to hold up Moshe’s hands and why was he left in charge of the nation along with Aharon while Moshe went to receive the Torah?

Rashi comments on Shmot 17:12 that Chur was Miriam’s son (this is derived from the Talmud, Sotah 11b).

In his commentary on Shmot 24:14, Rashi mentions that Chur was Miriam’s son and adds that his father was Kalev ben Yefuneh as it says in Divrei HaYamim I 2:19, “Kalev took Efrat (as a wife) and she gave birth to Chur. In Sotah 11b we learn that Efrat is Miriam.

The background story can be found in Divrei HaYamim 18-20:

And Kalev the son of Hetzron had children with Azuva, his wife and with Yeriot; her sons are Yesher and Shovav and Ardon. And when Azuva died Kalev married Efrat who gave birth to Chur. And Chur begot Uri and Uri begot Bezalel.

The Agaddeta in the Talmud, Masechet Sotah answers some difficult questions on these verses:

Why is Kalev called the son of Hetzron if in the Torah his name is Kalev the son of Yefuneh?

Kalev’s father’s name was Hetzron. However, since he turned aside (fanah) from the bad behavior of the other scouts, he was called the son of Yefuneh.

Who were Azuva and Yeriot?

They were both names of Miriam. She was called Azuva (deserted) since the men didn’t want to marry her when she had the skin disease, tzaraat. She was called Yeriot since her complexion was similar to undyed curtains.

Why don’t we know anything about Yesher and Shovav and Ardon?

They weren’t actually sons (vaneha), rather they were her builders (voneha) and describe her husband, Calev who didn’t follow the bad behavior of the scouts who gave about report about the Land of Israel.

Why does it say that she died?

When she was sick it was almost like she died.

Why is she called Efrat?

The name Efrat has the root “paru”, “were fruitful.” When Miriam was a midwife (Puah) she ensured that B’nai Yisrael were fruitful and multiplied.

Why does it say that Calev married Efrat?

When she was healed it was like she was a new person and they renewed their wedding vows.

King David was called an Efrati since he was a descendant of Miriam.

Chur was the son of important parents, Miriam and Kalev, leaders of B’nai Yisrael. His uncles were Moshe and Aharon. He merited to have Bezalel, the artist chosen to create the artifacts for the mishkan (Tabernacle), as his grandson.