Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
When making duaa for others in without their presence, the angels will respond to your duaa and say “and the same onto you.”
Sheikh Suleiman Hani answers your question in the following video:
What is the benefit of praying for others in their absence?
Imam Muslim reports in hadith in which the prophet (sallallahu alayhi wasalam) informs us about the reward of the person who prays for his brother behind his back (without his presence).
When you are asking Allah on behalf of someone for forgiveness or for goodness in this life or the next, what is the benefit for you and I?
The benefit is that the Angels will respond to your duaa and they will say, “And the same unto you,” meaning what? You will also have a benefit from this dua.
One of the greatest things you can do for yourself and for others is to constantly include other people in your dua. This especially starts with your parents; this includes people who have a right upon you like your spouse, your children, and most importantly the entire Ummah.
Why are you making duaa for everyone else?
Imagine making duaa for a billion people and the response on your behalf from the Angels is for a billion different types of rewards or blessings, and so on and so forth.
Make duaa for others sincerely for their benefit, and you will find there is a benefit and increase in blessings in your life as well.
Allah subhana wa ta’ala knows best.
Almighty Allah knows best.
Source: www.faithiq.org