First Council of Nicaea, Canon I

If any one in sickness has been subjected by physicians to a surgical operation, or if he has been castrated by barbarians, let him remain among the clergy; but, if any one in sound health has castrated himself, it behoves that such an one, if [already] enrolled among the clergy, should cease [from his ministry], and that from henceforth no such person should be promoted.  But, as it is evident that this is said of those who wilfully do the thing and presume to castrate themselves, so if any have been made eunuchs by barbarians, or by their masters, and should otherwise be found worthy, such men the Canon admits to the clergy.

Translation: Henry R. Percival, The Seven Ecumenical Councils, NPNF II.14 (1900), via CCEL.