Hi, I don’t recall that precisely, but if you look at the discussion by Ven Thanissaro in BMC 2 (rev 2013) on Medicine, he points out scripture & commentary that describe practicing medicine as wrong livelihood for all bhikkhus:
The Great Section on Virtue in the S›maññaphala Sutta (DN 2) lists the types of wrong livelihood from which a bhikkhu should abstain. Among them is the practice of medicine, or in the words of the sutta:
“Administering emetics, purges, purges from above, purges from below, head-
purges; ear-oil, eye-drops, treatments through the nose, ointments, and counter-ointments; practicing eye-surgery (or: extractive surgery), general surgery, pediatrics; administering root-medicines and binding medicinal herbs—he abstains from wrong livelihood, from lowly arts such as these. This, too, is part of his virtue.”
The Commentary to Pr 3 states that a bhikkhu should not act as a doctor for lay people unless they are:
his parents, people who care for his parents, his other blood relatives;
his preceptor and teacher’s parents or other blood relatives;
applicants for ordination;
his own steward;
travelers who arrive ill at his monastery;
people who fall ill while in the monastery.
In none of these cases, however, should he expect material reward for his services.
Bhikkhus are, however, expected to know enough medicine to care for their own and for one another’s illnesses… >
The one profession I recall as forbidden only to monks who had practiced it in lay life is the case of barbers. Also from BMC2:
Mv.VI.37.5 tells the story of a former barber who had ordained late in life and still kept his barber’s equipment at hand. Giving his equipment over to his sons, who were also skilled barbers, he had them go from house to house taking the equipment along to ask for offerings of food. The boys were very successful. Donors, feeling intimidated by the razors, etc., gave donations even though they didn’t want to. As a result, the Buddha laid down a double rule: that a bhikkhu should not get others to do what is unallowable, and that one who was formerly a barber should not keep barber’s equipment. The first rule seems to mean that one should not get others to dissemble, talk, hint, etc., for the sake of material gain. The second rule seems related to the fear that people in those days had of barbers, who were reputed to be so skilled with their razors that they could kill without leaving a visible wound. Thus, to make sure that a bhikkhu who was formerly a barber cannot intimidate anyone, he should not have barber’s equipment at hand. The Commentary states that a former barber is allowed to use barber’s equipment (e.g., to shave the heads of his fellow bhikkhus) but is not allowed to keep it or to accept payment for using it. Other bhikkhus may keep barber’s equipment without offense.
Hope that is useful.
[Edited to fix typos]