For deceased - 1
اللَّهُمَّ اغْفِرْ لَهُ الَّلهُمَّ ثَبِّتْهُ
Allaahum-maghfir lahu Allaahumma thabbithu
O Allah, forgive him. O Allah, strengthen him.
Abu Dawud 3:315
Supplication
For deceased
2 duas with Arabic, transliteration and translation.
For deceased - 2
اللهُـمِّ اغْفِـرْ لِـ-فُلاَنٍ (باسـمه)وَارْفَعْ دَرَجَتََـهُ فِي المَهْـدِيّيـنَ ، وَاخْـلُفْـهُ في عَقِـبِهِ في الغَابِِـرِينَ، وَاغْفِـرْ لَنَا وَلَـهُ يا رَبَّ العـالَمـين، وَافْسَـحْ لَهُ في قَبْـرِهِ وَنَـوِّرْ لَهُ فِيهِ
Allaahummaghfir li (name of the person) warfa' darajatahu fil-mahdiyyeena, wakhlufhu fee 'aqibihi fil-ghaabireena , waghfir-lanaa wa lahu yaa Rabbal-'aalameena, wafsah lahu fee qabrihi wa nawwir lahu feehi
O Allah, forgive [name of the person] and elevate his station among those who are guided. Send him along the path of those who came before, and forgive us and him, O Lord of the worlds. Enlarge for him his grave and shed light upon him in it.
Muslim 2:634Each dua is shown with its reference where the source records one, so you can check it. Duas whose wording you intend to rely on in worship are worth confirming with a qualified teacher.