מדרש על אבות 5:6

"That God created to do/make." These are the ten things that were created on the eve of the Sabbath at twilight: [1] the mouth of the earth, [2] the mouth of the well, [3] the mouth of the donkey, [4] the rainbow, [5] the manna, [6] the letters, [7] the writing, [8] the tablets, [9] the grave of Moses and [10] the ram of Abraham, our father. [...] For all these God created to do miracles and mighty deeds. [...]

THE CREATION ON THE EVE OF THE SABBATH
TEN things were created (on the eve of the Sabbath) in the twilight (namely): the mouth of the earth; the mouth of the well; the mouth of the ass; the rainbow; the Manna; the Shamir; the shape of the alphabet; the writing and the tables (of the law); and the ram of Abraham. (Some sages say: the destroying spirits also, and the sepulchre of Moses, and the ram of Isaac; and other sages say: the tongs also.)