Abu al-Darda reported that the Prophet, peace be upon him, said:
“There is no Muslim worshipper who prays for his brother (Muslim) in his absence except that an angel says, ‘And to you the same!'” (Recorded in Muslim)
And Safwan ibn ‘Abdillah narrated: ‘I went to Syria, and visited Abu al-Darda in his home, but he was not present when I arrived. Umm al-Darda asked me: ‘Are you going to perform Hajj this year?’ I replied that I was, so she said, ‘In that case, do not forget to pray to Allah for us for good, for the Prophet, peace be upon him, used to say:
“The du’a of a Muslim for his brother in his absence is responded to. There is an angel in front of him that has been assigned to him; every time he makes a du’a for his brother with good, the angel assigned to him says, ‘Amin! And to you the same.'” (Recorded in Muslim)
In this situation, the du’a must be purely for the sake of Allah, since a Muslim will love his brother Muslim only for the sake of Allah, and this is the only factor what will cause him to make du’a for a brother in his absence.
It is reported that the pious predecessors would make du’a for their brothers when they themselves were in need of that du’a, since they knew that this type of du’a was responded to.
If we reflect on this point, we can see what effects true brotherhood and sisterhood would create within the community – where every Muslim was praying for other Muslims, for their good in this life and in the Hereafter.