Transliteration:
Nawaitu an asuma ghadan lillahi ta'ala min fardi Ramadan.

"I intend to fast tomorrow for the sake of Allah in the month of Ramadan."

Note: Intention (Niyyah) is made in the heart. This Arabic wording is commonly recited but not required verbally.

Transliteration:
Dhahaba adh-dhama'u wabtallati al-'urooq wa thabata al-ajru in sha Allah.

"The thirst has gone, the veins are moistened, and the reward is certain, if Allah wills."

Reference: Abu Dawud (2357) – Hasan

There is no specific authentic dua prescribed exclusively for Sehri. The intention in the heart is sufficient. However, you may make general dua and seek blessings before beginning the fast.

Important Sunnah: The Prophet ﷺ said:
"Take Suhoor, for indeed there is blessing in it." (Bukhari & Muslim)

The most common and authentic version:

Transliteration:
Allahumma salli 'ala Muhammadin wa 'ala aali Muhammadin kama sallayta 'ala Ibrahima wa 'ala aali Ibrahima innaka Hameedum Majeed. Allahumma baarik 'ala Muhammadin wa 'ala aali Muhammadin kama baarakta 'ala Ibrahima wa 'ala aali Ibrahima innaka Hameedum Majeed.

"O Allah, send Your grace and honour upon Muhammad and upon the family of Muhammad, just as You sent grace and honour upon Ibrahim and upon the family of Ibrahim. You are indeed Praiseworthy, Glorious. O Allah, send Your blessings upon Muhammad and upon the family of Muhammad, just as You sent blessings upon Ibrahim and upon the family of Ibrahim. You are indeed Praiseworthy, Glorious."

Virtue: Whoever sends one blessing upon the Prophet ﷺ, Allah sends ten blessings upon him. (Muslim)

The version often recited in the Witr prayer:

Transliteration:
Allahumma inna nasta'inuka wa nastaghfiruka wa nu'minu bika wa natawakkalu 'alaika wa nuthni 'alaikal khair, wa nashkuruka wa la nakfuruka wa nakhla'u wa natruku man yafjuruk. Allahumma iyyaka na'budu wa laka nusalli wa nasjudu wa ilaika nas'a wa nahfidu wa narju rahmataka wa nakhshaa 'adhabaka inna 'adhabaka bil kuffari mulhiq.

"O Allah, we seek Your help and ask for Your forgiveness, and we believe in You and rely on You. We praise You for all the good, and we are grateful to You and do not disbelieve in You. We renounce and forsake those who disobey You. O Allah, You alone we worship, to You we pray and prostrate, and towards You we strive and make haste. We hope for Your mercy and fear Your punishment. Indeed, Your punishment will certainly reach the disbelievers."

Note: Dua-e-Qunoot is often recited in the last rak'ah of the Witr prayer, especially during the second half of Ramadan in some traditions.

Transliteration:
Allahumma innaka ‘afuwwun tuhibbul ‘afwa fa’fu ‘anni.

"O Allah, You are Most Forgiving, and You love forgiveness, so forgive me."

Reference: Tirmidhi (3513) – Sahih