Exegesis:

ti eti echomen marturias chreian ‘what do we still need testimony?.’ No witnesses are needed any more.

marturia ‘testimony,’ here with passive force, ‘things said by witnesses in court,’ ‘testimony given in court.’

apo tou stomatos autou ‘from his mouth,’ cf. on 19.22.

Translation:

Said is reciprocal: they are speaking to each other.

What further testimony do we need, or, describing the noun, ‘what do we still need words/declarations of witnesses,’ ‘why should we still need to hear things that witnesses can tell us (or, that other people know) about him.’

Heard it … from his own lips, or, ‘mouth’; or, “heard his very own words” (Good News Translation, similarly Tae’ 1933), ‘heard him say it himself’ (cf. Batak Toba).

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.