The first few lines are?

I heard it from the valleys
I heard it ringing in the mountains
Ah ah ah ah ah ah ah

I heard it from the valleys
I heard him singing in the mountains
Ah ah ah ah ah ah ah

The rest of the lyrics are taken from the poem 'A Letter Home' by Siegfried Sassoon, written in May 1916.
This poem can be found in many editions of his work (we are not allowed to re-print it here).