虎穴に入らずんば虎子を得ず (hiragana こけつにいらずんばこじをえず, romaji koketsu ni irazunba koji o ezu)
Literally, “one may not catch a tiger cub without entering a tiger's lair.” From the Chinese text Book of the Later Han: 不入虎穴，不得虎子. Attributed to Ban Chao.