What the tradition hands you to say and do
Islam does not leave a bereaved person to invent a response. There is a sentence to say, and there is a report about what it is met with.
The Qur'an gives the words themselves in 2:156, and the report below carries a supplication attached to it. Whoever is struck by a calamity and says it, asking to be rewarded in the affliction and given something better in its place, is given something better. Then the report that for the believing servant whose beloved is taken and who bears it, there is a reward with no lesser name on it than Paradise.
And two passages that are not about being told to cheer up. 93:1 to 8 was revealed to somebody in a state of abandonment, and it does not argue with the feeling, it enumerates what was given. And 94:5 to 6 says that with hardship comes ease, twice. Ease with the hardship, not instead of it and not after it. This platform takes its name from that surah, and we will say that once and not again.
Qur’an and Sunnah
ٱلَّذِينَ إِذَآ أَصَٰبَتۡهُم مُّصِيبَةࣱ قَالُوٓاْ إِنَّا لِلَّهِ وَإِنَّآ إِلَيۡهِ رَٰجِعُونَ
those who say, when afflicted with a calamity, ‘We belong to God and to Him we shall return.’
Qur’an 2:156
وَٱلضُّحَىٰ
By the morning brightness
Qur’an 93:1
وَوَجَدَكَ عَآئِلࣰ ا فَأَغۡنَىٰ
Did He not find you in need and make you self-sufficient
Qur’an 93:8
فَإِنَّ مَعَ ٱلۡعُسۡرِ يُسۡرًا
So truly where there is hardship there is also ease
Qur’an 94:5
إِنَّ مَعَ ٱلۡعُسۡرِ يُسۡرࣰ ا
truly where there is hardship there is also ease
Qur’an 94:6
Narrated Abu Huraira:Allah's Messenger (ﷺ) said, "Allah says, 'I have nothing to give but Paradise as a reward to my believer slave, who, if I cause his dear friend (or relative) to die, remains patient (and hopes for Allah's Reward)
Sahih al-Bukhari 6424graded sound, sahihaynfull report
Umm Salama reported Allah's Messenger (ﷺ) as saying:If any Muslim who suffers some calamity says, what Allah has commanded him," We belong to Allah and to Him shall we return; O Allah, reward me for my affliction and give me something better than it in exchange for it," Allah will give him something better than it in exchange. When Abu Salama died she said: What Muslim is better than Abu Salama whose family was the first to emigrate to the Messenger of Allah (ﷺ). I then said the words, and Allah gave me God's Messenger (ﷺ) in exchange. She said: The Messenger of Allah (ﷺ) sent Hatib b. Abu Balta'a to deliver me the message of marriage with him. I said to him: I have a daughter (as my dependant) and I am of jealous temperament. He (the Holy Prophet) said: So far as her daughter is concerned, we would supplicate Allah, that He may free her (of her responsibility) and I would also supplicate Allah to do away with (her) jealous (temperament)
Sahih Muslim 918graded sound, sahihaynfull report
Translation: M.A.S. Abdel Haleem, The Qur'an, Oxford University Press, 2004.