What is the name of the piece of music Mozart wrote when he was dying?

the name of the piece of music he began to write when he started dying was called the Requiem and after a short amount of time he started to convince himself that this was his own death music. he continued on this piece of music until he died and never finished it. the last 3 sections of the Requiem were completely lacking when he died and one of his pupils Franz Sussmayer ended up finishing it.
He was just a couple of measures into another symphony.