Charak Samhita
कालयोगबलाः केचित् केचिदभ्यसनध्रुवाः। केचित् प्रयत्नैर्व्यज्यन्ते [२] वृषाः केचित् स्वभावतः ॥७॥
There are some persons who get their sexual vitality at a particular season, some get because of regular habit, some by taking virilific preparations, and some are virile by nature.
english translation
kAlayogabalAH kecit kecidabhyasanadhruvAH। kecit prayatnairvyajyante [2] vRSAH kecit svabhAvataH ॥7॥
hk transliteration by Sanscript