Tafsir of Al-Anbiya' 21:72

وَوَهَبْنَا لَهُ إِسْحَاقَ وَيَعْقُوبَ نَافِلَةً (And We blessed him with Ishaq and Ya` qub as gift - 21:72) That is, Allah Ta` la had bestowed upon him the son Ishaq (علیہ السلام) as per his supplication and in addition to that a grandson Ya` qub (علیہ السلام) as a gift. This is why he is referred to as نَافِلَةً nafilah which originally means 'extra'.