What is the meaning of "nāfilah" in His, the Almighty’s, saying: (And We bestowed upon him Isaac and Jacob as a nāfilah)? And what is the most likely meaning intended by "nāfilah": is it Isaac and Jacob together, or Jacob alone? And what are the proofs that support this view?

"Two interpretations concerning the meaning of 'al-nafilah' in the noble verse have been narrated from the Salaf (early Muslims), and the context of the verse accommodates both. The first is that 'al-nafilah' means an increase, and it refers specifically to Jacob, as if Abraham asked for one and was given two. The second is that 'al-nafilah' means a gift, and it refers to both Isaac and Jacob together. Al-Tabari mentioned that the verse can bear both meanings and that there is no stronger evidence to prefer one over the other."