The دعاء for protection from Hellfire is: 'Allahumma ajirni minan-nar' — O Allah, protect me from the Fire. النبي said whoever asks Allah for Paradise three times, Paradise says 'O Allah, admit him to Paradise,' and whoever seeks protection from the Fire three times, the Fire says 'O Allah, protect him from me.'
اللَّهُمَّ أَجِرْنِي مِنَ النَّارِ
Allahumma ajirni minan-nar.
Meaning: O Allah, protect me from the Fire.
Source: Sunan al-Tirmidhi #2572This simple yet powerful دعاء engages creation itself in interceding on behalf of the supplicant. النبي encouraged his companions to seek protection from the Fire frequently. Abu Hurayrah narrated that النبي said: 'When you pray, seek refuge with Allah from four things: the punishment of the grave, the punishment of the Hellfire, the trials of life and death, and the evil of al-Masih ad-Dajjal' (صحيح مسلم 588).
اللَّهُمَّ أَجِرْنِي مِنَ النَّارِ
The Prophet (peace be upon him) said: 'Whoever asks Allah for Paradise three times, Paradise says: O Allah, admit him to Paradise. And whoever seeks protection from the Fire three times, the Fire says: O Allah, protect him from the Fire.'