According to Menachem Zohari in his series of books on the sources of Rashi's commentary on Tanach, "Mekorot Rashi", in the book on Yechezkel, Rashi's sources for this commentary are:
The Talmud in Erchin 12a, as brought in the above answer by @AlBerko.
Two sections of Seder Olam Rabbah:
a. Chapter 11 (my translation):
"...and so it says &...
IIRC, the explanation goes like this:
There's no Biblical festival of R"H, there's one of blowing the Shofar on the first day of the 7th month.
On Jubilees, the main Shofar (to free the slaves, etc) is blown on the 10th day of the 7th month as it says:
"וְסָפַרְתָּ לְךָ שֶׁבַע שַׁבְּתֹת שָׁנִים שֶׁבַע שָׁנִים שֶׁבַע פְּעָמִים וְהָיוּ לְךָ יְמֵי ...