It’s from a poem written by Pavel Friedmann, in Theresienstadt concentration camp on June 4, 1942. He was later murdered in Auschwitz.
The full poem: (there are different versions because it is a translation)
The Last Butterfly
He was the last. Truly the last.
Such yellowness was bitter and blinding
That was his true colour.
And how easily he climbed, and how high,
Certainly, climbing, he wanted
To kiss the last of my world.
I have been here seven weeks,
Who loved me have found me,
Daisies call to me,
And the branches also of the white chestnut in the yard.
That last one was the last one.
There are no butterflies, here, in the ghetto.