सञ्जय
In plain words
Sanjaya said: Thus I heard this wondrous, hair-raising dialogue between Vasudeva and the Mahātma Partha.
Word for word
Thus I heard this wondrous dialogue, making the hair stand on end, of Vāsudeva and the great-souled Pārtha.
The commentators
Bhagavadgītābhāṣyam
परिसमाप्तः शास्त्रार्थः । अथ इदानीं कथासम्बन्धप्रदर्शनार्थं सञ्जयः उवाच —
इति एवम् अहं वासुदेवस्य पार्थस्य च महात्मनः संवादम् इमं यथोक्तम् अश्रौषं श्रुतवान् अस्मि अद्भुतम् अत्यन्तविस्मयकरं रोमहर्षणं रोमाञ्चकरम् ॥ ७४ ॥
In plain words
The teaching of the śāstra is finished; its whole meaning has been set down. Now, to show where the story takes up again, Sañjaya speaks—
"Thus" means: in just this way. I have heard this conversation between Vāsudeva and Pārtha, the great-souled one, exactly as it was spoken — wondrous, meaning it produces the utmost astonishment; hair-raising, meaning it makes the hair stand up on the body.
Close to the Sanskrit
Completed is the meaning of the scripture. Now, at this point, for the purpose of showing the connection of the narrative, Sañjaya said—
'Thus' — that is, in this manner — I heard, I have heard, this conversation of Vāsudeva and of Pārtha the great-souled one, as it was spoken; wondrous, that is, producing extreme astonishment; hair-bristling, that is, causing horripilation.