He [Imam Khomeini] was accustomed to MafÄtīḥ al-JinÄn1, even when he was hospitalized.
When he passed away and we brought his MafÄtīḥ home,
we understood that he had written a start date -8th of Shawwal- next to the ‘Ahad2 prayer. He used to recite this prayer for forty mornings…
1-    Mafatih al-Jinan by QummÄ« is a Twelver Shi’a compilation of supplications narrated from the Ahl al- Bayt (pbu th), prayers, the texts of pilgrims, etc. The book is widely popular in the Twelver world.
2-    Ahad prayer is the one, Imam SÄdiq (pbuh) narrated that “One who recites this supplication for 40 mornings; he will be accounted amongst the helpers of Imam MahdÄ« (mgehr). And if he happens to die before the reappearance of Imam MahdÄ« (mgehr), Allah will raise him up from his rave so that he may assist the holy Imam (mgehr).For every word that he recites in this supplication, Allah will grant him 1000 good deeds and will erase from his scroll of deeds 1000 sinsâ€.
Some notes of Ayatullah [Imam] Khomeini ‘s style , Vol. 3, P. 43