I hope this email finds you in the best of health.
1) Couple of days ago I committed such a disgraceful sin which from my perspective is unforgivable. I attributed to Allah something which he is free from, I stated that Allah made a mistake (As-tagfiruallaah) and that Jabriel was only a myth. Out of fear of Allah I have been trembling and worried about what I said. I have repented to Allah I have even cried during Du'aa'. I don't think I will be able to bear Allaah's punishment so could you please tell me any specific Du'aa' or verse which would help me in my repentance.
2) Could you please tell me where Amana r-Rasoolullaah... (The Messenger believes...) is in the Qur'aan and where the passage ends.
Dear brothers could you provide me with an answer very quickly, because of my foolish mistake I have fallen prey to unnecessary depression I truly fear Allaah's punishment may over take me. So could you provide me with an answer.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

No doubt, what you have pronounced is a major disbelief according to the consensus of Muslim scholars. Nevertheless, there is no sin that cannot be overcome with a sincere repentance. If one makes sincere repentance, Allaah accepts it due to His great Generosity and Forgiveness. Allaah says (interpretation of meaning): {And it is He who accepts repentance from His servants and pardons misdeeds, and He knows what you do.}[42:25].

Allaah also says (interpretation of meaning): {And whoever does a wrong or wrongs himself but then seeks forgiveness of Allaah will find Allaah Forgiving and Merciful.}[4:110].

Ibn 'Abbaas said: "Some polytheists killed innocent people in a great number, committed fornication and adultery much and then they came to the Prophet and said: "Really your teachings and sayings are good, but can you inform us how could our major bad deeds be expiated?" Then Allaah revealed this verse (interpretation of meaning): {And those who do not invoke with Allaah another deity or kill the soul which Allaah has forbidden [to be killed], except by right, and do not commit unlawful sexual intercourse. And whoever should do that will meet a penalty. Multiplied for him is the punishment on the Day of Resurrection, and he will abide therein humiliated. Except for those who repent, believe and do righteous work. For them Allaah will replace their evil deeds with good. And ever is Allaah Forgiving and Merciful}[25:68-70].

Allaah also reveals the verse (interpretation of meaning):{Say, “O My servants who have transgressed against themselves [by sinning] do not despair of the mercy of Allaah. Indeed, Allaah forgives all sins (1263). Indeed, it is He who is the Forgiving, the Merciful.}[39:53].

The Prophet said: "One who repents from his sin is just as if he has never sinned". [Ibn Maajah]

Imaam Muslim reported from Abu Hurayrah that the Prophet said: "If you had not committed sins, Allaah would have perished you and would have replaced you with a nation that commits sins and asks Allaah's forgiveness, and ultimately Allaah forgives it."

As for the supplication you are asking about it is called 'Sayyidul Istighfar' which is: "Allaahumma Anta Rabbee laa Ilaaha illaa Anta Khalaqtanee wa Ana 'Abduka wa Ana 'Ala 'Ahdika wa w'adika mastata'tu, A'uoothu bika min Sharri ma sana'tu Aboo'u laka bini'amatika 'Alayya wa Aboo'u bithanbi faghfirlee, fa innahu laa yaghfirutthnooba illa Anta." The meaning of this supplication is, (O Allaah! You are my Lord, none has the right to be worshipped except You. You created me and I am Your servant and I abide by Your covenant and promise as best as I can. I take refuge in You from the evil which I have committed. I acknowledge Your favor upon me and I acknowledge my sin, so forgive me, for verily none can forgive sin except You." [Al-Bukhari]

Imaam At-Tirmithi reported from Ibn 'Umar that the Prophet used to say:  "Rabbighfirlee wa tub 'Alayya innaka Antat-Tawwaabul ghafoor" hundred times in one gathering. The meaning of this supplication is "O my Lord, forgive me and accept my repentance. Verily You are The Oft-forgiving, The Most Merciful."

Therefore, you have to ask forgiveness of Allaah as much as possible, multiply your good deeds, especially giving charity in secret since it protects against the wrath of Allaah as reported by the Prophet .

The verse you are asking about is chapter Al-Baqarah, verse: 285 [2:285].

Allaah knows best.