Charak Samhita
विद्यासमाप्तौ ब्राह्मं वा सत्त्वमार्षमथापि वा। ध्रुवमाविशति ज्ञानात्तस्माद्वैद्यो द्विजः स्मृतः ॥५३॥
On completion of training, Brahma or Arsha psyche enters in to him certainly according to the knowledge. Hence the physician is known as the thrice born.
english translation
vidyAsamAptau brAhmaM vA sattvamArSamathApi vA। dhruvamAvizati jJAnAttasmAdvaidyo dvijaH smRtaH ॥53॥
hk transliteration by Sanscript