Vidui - Confession

Leviticus 5:5; 26:40
Numbers 5:6-7

The need and obligation to repent

Deuteronomy 4:30-31; 30:1-10
Isaiah 55:6 etc.
Jeremiah 3:14, 35:15, etc.
Ezekiel 18; 33:11-20, etc.
Hosea 14:2-3
Joel 2:12-13
Amos 4:6,8,10,11
Micah 7:18-19
Zephaniah 2:1-3
Haggai 2:15
Zechariah 1:3-4
Malachi 3:7
Lamentations 5:21

Instances of Teshuvah - repentance

Genesis 35:2-4
Judges 10:10-16
1 Samuel 7:36
1 Kings 21:27-29
2 Kings 22:11;19-20
Jonah 3:5-10;4:10-11

The Story of Gedaliah

2 Kings 25:22-26
Jeremiah 40-43

Post-biblical sources

Maimonides, Mishneh Torah, Laws of Repentance, 1-6
Shulhan Aruch, Orah Hayim, (and Mishnah Berurah) §§ 602-603
Kitzur Shulhan Aruch § 130