Allah, the Most High, has described the virtue of patient believers in the Quran, saying:
الَّذِيْنَ اِذَاۤ اَصَابَتْهُمْ مُّصِيْبَةٌ قَالُوْۤا اِنَّا لِلهِ وَ اِنَّاۤ اِلَيْهِ رٰجِعُوْنَ.

“Give good tidings to the patient, who, when disaster strikes them, say, 'Indeed we belong to Allah, and indeed to Him we will return.'" - Surah Al-Baqarah (02): 156

Many of us think that when we hear news of someone's death, we should say 'Inna lillahi wa inna ilaihi raji'un.' However, the matter is not like this. It is understood from the context of the verse that a believer should recite ‘inna lillhi wa inna ilaihi raji’un’ whenever he faces any adversity. The essence of the verse is to turn towards Allah.

A believer turns towards Allah during all of his difficulties. In any adversity, they seek solace from the Almighty Allah. That's why the noble Prophet (peace be upon him) and the noble companions (may Allah be pleased with them) used to recite 'Inna lillahi wa inna ilaihi raji'un' in any calamity. For these patient believers, the promise of great rewards has been announced in the following verse:

"Those are the ones  upon whom there are blessings from their Lord and mercy as well; those are the ones who are on the right path." - Surah Al-Baqarah (02): 157"