Jesus used this phrase to refer to himself. This title also referred to the Christ, but the elders would have to ask to verify that he was claiming to be the Christ. (See: [[rc://en/ta/man/translate/figs-123person]])
The council asked this question because they wanted Jesus to explicitly confirm their understanding that he was saying he was the Son of God. AT: "So when you said that, did you mean that you are the Son of God?" (See: [[rc://en/ta/man/translate/figs-explicit]])
The phrase "his own mouth" refers to his speech. AT: "heard him say that he believes he is the Son of God" (See: [[rc://en/ta/man/translate/figs-metonymy]])