الكافي 2/2/30/3

3. It is narrated from him (narrator of the Hadith above) from Ahmad ibn Muhammad from Ali ibn al-Hakam from Sayf ibn ‘ Amirah from ‘Amr ibn Shimr from Jabir from abu Ja’far عليه السلام who has said the following: “About the words of Allah, the Most Holy, the Most High, ‘He answers the prayers of the righteously striving believers and grants them increasing favors . . . ,’ (42:26) the Imam said, ‘This is a reference to a believing person who prays for his brother (in belief) in his absence and the angel says, “Amin.” Allah, the Majestic and the Most dominant says, ‘You will have twice as much you asked for you brother (in belief) and I have already granted him what you asked for due to your love for him.’”