Qur'anic Dua - 20 (English)
رَّبِّ ارْحَمْهُمَا كَمَا رَبَّيَانِي صَغِيرًا
Lord, have mercy upon both of them (parents) as they nourished me when I was a child.
This dua is mention in Surah Isra, Ayah No. 24.
In ayah 23 of this surah, Allah almighty commanded us to be kind to our parents. If they reach old age, we should say no word that displeases them, and we should not be harsh with them, but speak to them respectfully always. And in Ayah 24 Allah instructed us to be humble with them out of mercy, and pray with the above supplication.
We should always pray for our parents because their favours in our lives are as much as that we can not count them.