It was narrated from Fatimah bint Husain that her father said: The Prophet (ﷺ) said: “Whoever was stricken with a calamity and when he remembers it he says ‘Inna lillahi, wa inna ilayhi raji’un (Truly, to Allah we belong and truly, to Him we shall return),’ even though it happened a long time ago, Allah will record for him a reward like that of the day it befell him.”
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا وَكِيعُ بْنُ الْجَرَّاحِ، عَنْ هِشَامِ بْنِ زِيَادٍ، عَنْ أُمِّهِ، عَنْ فَاطِمَةَ بِنْتِ الْحُسَيْنِ، عَنْ أَبِيهَا، قَالَ قَالَ النَّبِيُّ ـ صلى الله عليه وسلم ـ " مَنْ أُصِيبَ بِمُصِيبَةٍ فَذَكَرَ مُصِيبَتَهُ فَأَحْدَثَ اسْتِرْجَاعًا - وَإِنْ تَقَادَمَ عَهْدُهَا - كَتَبَ اللَّهُ لَهُ مِنَ الأَجْرِ مِثْلَهُ يَوْمَ أُصِيبَ " .
sunnah.com