رَّبِّ ارْحَمْهُمَا كَمَا رَبَّيَانِي صَغِيرًا. My Lord, have mercy on them, as they raised me when I was a child. (Quran 17:25) You can make supplications for them for every good thing, but the above dua of the Quran is a very comprehensive one, which have been revealed directly by Allah Himself. In the dua, Allah has asked us to pray for His mercy (for them) since this is very important to them. Experience has shown that when parents become old, they really do not turn towards comfort, luxury, pleasure and enjoyment. Their days of pleasure and enjoyment have gone with youth and maturity, and as they grow older they look forward for love, care, attention, compassion and mercy. As such, when we (as children) pray for Allah’s mercy, then Allah will take care of them in the best possible manner.
