Thou must be true thyself,
If thou the truth wouldst teach;
Thy soul must overflow, if thou
Another's soul would'st reach!
It needs the overflow of heart
To give the lips full speech.

Think truly, and thy thoughts
Shall the world's famine feed;
Speak truly, and each word of thine
Shall be a fruitful seed;
Live truly, and thy life shall be
A great and noble creed.

Horatius Bonar

http://www.poemhunter.com/poem/be-true/