Dua when someone lost his close relatives

Invocation for the bereaved

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إِنَّ للهِ ما أَخَذ، وَلَهُ ما أَعْـطـى، وَكُـلُّ شَيءٍ عِنْـدَهُ بِأَجَلٍ مُسَـمَّى.فَلْتَصْـبِر وَلْتَحْـتَسِب

162. 'Inna lillaahi maa 'akhatha, wa lahu maa 'a'taa, wa kullu shay'in 'indahu bi'ajalin musamman . . . faltasbir waltahtasib .

Surely , Allah takes what is His , and what He gives is His , and to all things He has appointed a time ... so have patience and be rewarded.¹

Also good to say:

 

أَعْظَـمَ اللهُ أَجْـرَكَ، وَأَحْسَـنَ عَـزاءَ كَ، وَغَفَـرَ لِمَـيِّتِكَ

'A'dhamallaahu 'ajraka, wa 'ahsana 'azaa'aka wa ghafara limayyitika.

May Allah magnify your reward, and make perfect your bereavement, and forgive your departed.²

Reference:
¹ Al-Bukhari 2/80, Muslim 2/636.
²An-Nawawi, Kitabul-'Athkar, p. 126.