וַיָּבֹ֥א אֶל־אָבִ֖יו וַיֹּ֣אמֶר אָבִ֑י וַיֹּ֣אמֶר הִנֶּ֔נִּי מִ֥י אַתָּ֖ה בְּנִֽי׃
And he came unto his father, and said: ‘My father’; and he said: ‘Here am I; who art thou, my son?’
Radak on Genesis
ויבא אל אביו, into the room where he was lying down. He called to him: אבי, seeing he (Yitzchok) could not see him.
Radak on Genesis
?מי אתה, seeing that he had addressed him simply with the word אבי, he had doubts about whose voice he had heard he had to ask: “who are you?”