[priskern]

Originally Posted by Tenoia

Spoiler for From LN:

It's never specified. However, from the context in the LN, it is highly likely that he whispered the name of Kayaba Akihiko as he looked at Aincrad.

Erm no, actually it's the very same sentence he said when he enter SAO official on the first day of SAO in first episode. A.k.a, "I had return, to this world."