FROM WIKI

The Book of Jasher (also spelled Jashar; Hebrew: סֵפֶר הַיׇּשׇׁר sēfer hayyāšār), which means the Book of the Upright or the Book of the Just Man, is a lost book mentioned in the Hebrew Bible, often interpreted as a lost non-canonical book. Numerous forgeries purporting to be rediscovered copies of this lost book have been written. A different interpretation identifies it as a reference to the Pentateuch, specifically the Book of Genesis, an interpretation which is notably favored by the Jewish scholar Rashi in his commentary on the Hebrew Bible (see here-below his commentary on Joshua).

According to the Book of Joshua, while Joshua was winning a battle against Adonizedek (king of Jerusalem) and his allies, Joshua prayed for the sun and moon to stand still.[1] Joshua 10:13 then states:

And the Sun stood still, and the Moon stayed,
until the people had avenged themselves on their enemies.
Is this not written in Sefer HaYashar?— Joshua 10:13

The presence of this event in a book of poetry has been interpreted as a poetic description of the prolonged battle.[2]

According to the medieval Jewish scholar Rashi, "Sefer HaYashar" in this verse refers to the Pentateuch: Jacob's prophecy regarding Joshua's ancestor Ephraim—"His seed will fill the nations"[3]—was fulfilled when Joshua's victory gave him renown among the various nations who heard of the victory.[4]