In the Testament of Issachar, who was the fifth son of Jacob/Israel, Issachar gives some detail behind his conception, bought at the price of a couple mandrakes, and then goes into detail about how sinless he was and recommends that his progeny live the same way.
Issachar gathered his sons and explained how he was born: Rachel stole two mandrakes from Leah's son Reuben. When Leah asked for them back, Rachel refused, saying that she would permit Leah to have sex with Jacob/Israel that night in return for the mandrakes. Leah balked, saying that she herself was Jacob/Israel's first wife, but Rachel said that Jacob/Israel only married Leah because he was tricked. So Jacob/Israel had sex with Leah that night, and she conceived Issachar (1:1-17).
God told Jacob/Israel that Rachel would bear two children and Leah would have six children because Rachel had abstained from sex for two nights. Otherwise, Rachel would have been barren and Leah would have had eight children. (1:18-21). Instead of eating the mandrakes, Rachel had offered them to God's priest, which is why God blessed Rachel with children (1:22-23).
Issachar grew up to be a righteous man, always doing right, with a complete focus on pleasing God. God richly blessed him and helped him maintain his focus. From his prosperity, he generously gave to the oppressed and the poor (1:24-31).
Issachar advises his sons to completely focus (as he put it, having "singleness of your heart") on pleasing God. They can do this through righteous actions, such as not coveting money, avoiding vanity, yearning for the will of God, avoiding envy, malice, and worry, shunning evil, obeying God's Law, loving God and your neighbor, working diligently, and offering God the firstfruits of your labor (1:32-41).
Jacob/Israel had blessed Issachar with the firstfruits. Issachar's progeny are to obey Levi and Judah, for God gave them the priesthood and the kingdom respectively, and Gad will defend the kingdom (1:42-44).
Issachar knows that in the "last days" his progeny will forsake God for Beliar (Belial), so God will send them into exile. So they should tell their children God's commands, because if they repent God will deliver them back to their land (2:1-4).
At 126 years old, Issachar has not sinned. He has perfectly obeyed God's will, such as by avoiding fornication, wine, covetousness, guile, and lies, being sympathetic to the sorrows of others, giving to the poor, working godliness, and loving God and every man with all his heart (2:5-14). If his progeny will do these things, they will not succumb to Beliar's spirits, wicked men, or wild beasts (2:15-16).
Then Issachar died while still perfectly healthy (2:17-18).
Like Reuben, the Testament of Issachar does not contain much in terms of specific Christian prophesy. However, there are some strong correlations with New Testament writings.
The general message is having a singleness of mind focused on doing God's will (avoiding sin), which is pretty much the theme of most of the Bible, but explicitly referenced in John 7:17, John 9:31, Romans 12:2, Ephesians 6:6, 1 Thessalonians 4:3, Hebrews 10:7, Hebrews 10:36, 1 Peter 2:15, 1 Peter 3:17, 1 Peter 4:2, 1 Peter 4:19, and 1 John 2:17.
When Issachar speaks of this focus on God's will, many specific commandments are mentioned which, in one way or another, all get referenced in the New Testament, and the rest of the Old Testament for that matter. However, there are a few points which stand out as particularly well correlated to the New Testament presentation; adultery (referenced as the more-general fornication), loving your neighbor, and care for this life.
Issachar 1:28 says that he never thought about pleasure with women, and Issachar 1:35 and Issachar 2:6 both assert that avoiding looking at beautiful women prevents mental fornication; teachings uniquely consistent with Jesus' words in Matthew 5:27-28.
Good treatment of your neighbor is a repeated theme in this Testament, as you find in Issachar 1:26, Issachar1:33, Issachar 1:38, Issachar 2:8, and Issachar 2:14. The strongest parallel comes from Issachar 1:33 and Issachar 2:14, which state "love the Lord and your neighbour" and "I loved the Lord; likewise also every man with all my heart." respectively, which is essentially the same as Jesus' words in Matthew 22:37-39, Mark 12:29-31, Mark 12:33, and Luke 10:27, and is found later in Romans 13:9, Galatians 5:14, and James 2:8.
Issachar 1:34 states that God-focused men give greater importance on fulfilling God's will that preserving their own life, which correlates well to Jesus' words regarding those who lose their lives for Jesus' sake in Matthew 10:39, Matthew 16:25, Mark 8:35, Luke 9:24, Luke 17:33, and John 12:25.
Issachar does not reference any future kingdom, Salvation, or eternal
reward or punishment, but rather simply references that God gave Levi
the priesthood and Judah the kingdom in Issachar 1:43.
Issachar 2:15 says that if you obey God, Belair's evil spirits will flee from you, which is very similar to James 4:7.
Finally, speaking of Beliar, in Issachar 2:2 and Issachar 2:15 we see Beliar cast as opposite to God similar to the way in which Christianity claims Satan is opposite to God.
"Then appeared to Jacob an angel of the Lord, saying: Two children shall Rachel bear, inasmuch as she hath refused company with her husband, and hath chosen continency. And had not Leah my mother paid the two apples for the sake of his company, she would have borne eight sons; for this reason she bare six, and Rachel bare the two: for on account of the mandrakes the Lord visited her. For He knew that for the sake of children she wished to company with Jacob, and not for lust of pleasure." - Issachar 1:18-20
"Therefore, when I was thirty-five years old, I took to myself a wife, for my labour wore away my strength, and I never thought upon pleasure with women; but owing to my toil, sleep overcame me." - Issachar 1:28 (I'm guessing that he had a very mechanical marriage)
"And the Lord increased ten thousandfold His benefits in my hands; and also Jacob, my father, knew that God aided my singleness." - Issachar 1:30 (God helped him be sinless)
"[A single-minded man] doth not desire to live a long life, but only waiteth for the will of God." - Issachar 1:34
"And the spirits of deceit have no power against him, for he looketh not on the beauty of women, lest he should pollute his mind with corruption." - Issachar1:35 (the beauty of women corrupts the mind)
"Keep, therefore, my children, the law of God, and get singleness, and walk in guilelessness, not playing the busybody with the business of your neighbour, but love the Lord and your neighbour, have compassion on the poor and weak." - Issachar 1:38
"And leaving guilelessness, will draw near to malice; and forsaking the commandments of the Lord, they will cleave unto Beliar." - Issachar 2:2
"Except my wife I have not known any woman. I never committed fornication by the uplifting of my eyes." - Issachar 2:6 (equates lustful glances with fornication)
"And I shared my bread with the poor." - Issachar 2:12
"I loved the Lord; likewise also every man with all my heart." - Issachar 2:14
"So do you also these things, my children, and every spirit of Beliar shall flee from you, and no deed of wicked men shall rule over you;" - Issachar 2:15