Shiko, one of Basho's disciples, in Kuzu no Matsubara, a collection of his critical essays, gives an account of circumstances surrounding Basho's writing his most famous poem:
old pond
a frog jumps in
the sound of water
This poem was written by our master on a spring day. He was sitting in his riverside house in Edo, bending his ears to the soft cooing of a pigeon in the quiet rain. There was a mild wind in the air, and one or two petals of cherry blossoms were falling gently to the ground. It was the kind of day you often have in late March--so perfect that you want it to last forever. Now and then in the garden was heard the sound of frogs jumping into the water. Our master was deeply immersed in meditation, but finally he came out with the second half of the poem, "Frog jumps in/Splash!" [actually, 'sound of water' - see below] Our master thought for a while, but finally he decided on "Old pond."
NOTES
source The translation has been modified to account for the absence of ideophones [also see PDF] as we understand them, in Japanese. For an interesting and somewhat exhaustive discussion of Basho's frog haiku, click on name below . . .