In Islam, Inna lillahi wa inna ilayhi raji'un is part of the Quran, Sura Al-Baqara, Verse 156. The full verse is as follows:
Inna lillahi wa inna ilayhi raji'un, when a misfortune overtakes them, say: 'Surely we belong to Allah and to Him shall we return'."
This is the phrase that Muslims recite when a person is struck by calamity in life, and is usually recited upon hearing the news of someone's death. This can also be recited in any situation involving risk of any sort. The phrase is commonly translated as "Verily we belong to God, and to God we return."
Muslims believe that God is the One who gives and that it is He who takes away; He is testing humankind. Hence, a Muslim submits to God and is grateful and thankful to God for whatever they receive. On the other hand, they are patient and say this expression in times of turmoil and calamity.
Inna lillahi wa inna ilayhi raji'un
Sura Al-Baqara, Verse 156