A53 : by Tony Garland 
Yes: the Scriptures. 
I say this because the Bible is a Jewish document, both the OT and NT. With the probable exception of Luke, the concept of 'remnant' found throughout Scripture is set forth by Jewish authors.
Just a sampling of the many passages concerning the believing remnant within Israel includes: 1Ki 19:18; 2Ki 19:4; 2Ki 19:30; 2Ki 21:14; 2Ki 25:22; Ezr 9:8; Isa 1:9; Isa 6:13; Isa 7:3; Isa 10:20-22; Isa 28:5; Isa 37:4; Isa 37:31-32; Isa 46:3; Isa 59:21; Isa 65:8; Jer 5:10; Jer 5:18; Jer 23:3; Jer 50:20; Eze 5:3; Eze 6:8-10; Eze 9:8; Eze 9:11; Eze 11:13; Eze 14:22; Joe 2:32; Mic 2:12; Mic 7:18; Zec 11:10; Zec 13:8-9; Ro 9:6; Ro 9:27; Ro 11:5; Ro 11:17; Ro 11:25; Ga 6:16; 1Pe 1:1; Re 12:17
To gain more insight into the New Testament teaching concerning the Jewish remnant (the 'true Jews' or 'Israel of God'), you may find our course on Romans 9-11a to be helpful. Especially our commentary concerning Romans 9:6, 27; 11:5, 25.
You can also find an excellent treatment of the believing Jewish remnant by the Jewish writer John (Rev. 12)! We teach on Revelation 12 in our Revelation Courseb and the companion Revelation Commentaryc.
As to teaching concerning the doctrine of the remnant from a modern Jewish perspective, I can recommend materials from Ariel Ministriesd by Dr. Arnold Fruchtenbaum. You may also be interested in his thorough treatment of all aspects of Israel in the Scriptures: Israelology: The Missing Link in Systematic Theologye.
Scripture reveals that God, by His sovereign power, has reserved a core of believing Jews down through the ages. This 'remnant' is the spiritual seed through which the promised restoration of Israel will eventually spring forth (Rom. 11:25-29).
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