מדרש אויף בראשית 49:24

Opposition 14) And a time to gather stones - corresponding to And He led Israel through its midst, and He brought them to Mount Sinai and gathered them and stationed them around Mount Sinai, as it says "And you shall set bounds for the people round about" (Exodus 19:12), and Israel are called "stones" as it says "From there is the Shepherd, the Stone of Israel" (Genesis 49:24), "The stone despised by the builders" (Psalms 118:22), and they received the Torah which is life forever - for His kindness endures forever.

"and his hands were steadfast until the sun set": We are hereby apprised that he was (occupied with prayer and) fasting (until sunset). These are the words of R. Yehoshua. R. Eliezer Hamodai sys: The sin (of Israel) weighed down Moses' hands and he could not support them. What did he do? He turned to the deeds of the patriarchs, viz. "and they took a stone and they placed it beneath him": "and they took a stone" — the deeds of the patriarchs (viz. Genesis 49:24). "and he sat upon it (feminine) — the deeds of the matriarchs. ...

And the large rock. This is the Shechina, as it is stated, "from there the Shepherd, the Rock of Israel." (Genesis 49:24)