Hazrat Moulana Muhammad Ilyaas (rahmatullahi ‘alaih) once mentioned:
When one makes du‘aa for any Muslim in his absence, then in reality one is actually making du‘aa for oneself. The hadeeth has mentioned that when any person makes du‘aa for his Muslim brother to be blessed with goodness and success, then the angels of Allah Ta‘ala say to him, “May you be blessed with the same!” (i.e. O servant of Allah! May Allah grant you the very thing that you are making du‘aa for your brother to receive). Hence, to make du‘aa for every Muslim to be blessed with goodness is a definite method for one to secure the du‘aa of the angels. (Malfoozaat Hazrat Moulana Muhammad Ilyaas (rahmatullahi ‘alaih) pg. 110)
Source: Ihyaauddeen.co.za