Exodus 8:5
4 Frogs will come up against you, your people, and all your servants.”’” 5 The Lord spoke to Moses, “Tell Aaron, ‘Extend your hand with your staff over the rivers, over the canals, and over the ponds, and bring the frog* up over the land of Egypt.’” 6 So Aaron extended his hand over the waters of Egypt, and frogs came up and covered the land of Egypt.
Onkelos Exodus 8:1
1 And the Lord said to Mosheh, Say to Aharon, lift your hand with your staff upon the streams, upon the rivulets, and upon the lakes; and the frogs shall come up on the land of Mizraim. 2 And Aharon lifted up his hand over the waters of the Mizraee, and the frogs came up and covered the land of Mizraim. 3 And the magicians did so with their enchantments, and made frogs to come up on the land of Mizraim.
Sanhedrin 67b
Babylonian TalmudIt is stated with regard to the plagues of Egypt: “And the frog came up and covered the land of Egypt” (Exodus 8:2). Noting that the term “the frog” is written in the singular, Rabbi Elazar says: At first it was one frog; it spawned and filled the entire land of Egypt with frogs.
Notes and References
"... 'Frogs' ... The biblical reading is 'bring up the frog' (in the singular), suggesting that one frog covered the entire land. Indeed, Rashi cites an opinion found in the Babylonian Talmud (Sanhedrin 67b) and the Midrash (Exodus Rabbah 104) that a single frog came, split into other frogs and swarmed over Egypt. Our targumist prefers to interpret the biblical singular as 'frogs,' which is closer to the intended meaning. Rashi also states that the singular form represents a swarm of frogs, just as the word kinam in verses 13 and 14, in the singular, refers to many lice. Our targumist translates kinam in the plural in keeping with his understanding of the intended meaning of the word ..."
Drazin, Israel, and Stanley M. Wagner Onkelos on the Torah, Exodus: Understanding the Bible Text (p. 43) Gefen, 2006