Wrong fluid
Hippopotamus saliva will do NOTHING for muscle growth. Hippo semen, however, is full of test and other growth factors. It's best absorbed rectally. Here's how you do it:
1. Fly to Africa.
2. Rent a female hippo costume.
3. Wearing the costume, loiter near a hippo watering hole and from time to time sort of wag your rump back and forth.
4. If you're lucky, the hippo bulls will come running.
5. Warning: do not "inject" more than five hippo loads per day ... not because you'll get too much test, but because -- after that much hot hippo love -- you may require reconstructive surgery.