This is actually the slow movement from his First Symphony, the one that famously uses Frère Jacques (or Bruder Jakob, or Bruder Martin as Mahler called it) set in the minor mode.
Mahler revised it many times, adding and removing bits over the years. But he never quite got around to making an all-viola version. He probably got distracted by the sound of all those cows clanging away on their bells outside his window. Understandable, really. It also features his characteristic Klezmer sounds, reminiscent of his Jewish childhood.
It’ll be a great play, ideal for moderate abilities upwards, with lots of melodic interest in all the parts. 10 mins. M