Michael Haydn's Symphony No. 1 in C major, Perger 35, Sherman 1, MH 23, is believed to have been written in Vienna around 1759. It is not clear whether this is the first symphony Michael Haydn wrote.
Charles H. Sherman and T. Donley Thomas, Johann Michael Haydn (1737 - 1806), a chronological thematic catalogue of his works. Stuyvesant, New York: Pendragon Press (1993)
C. Sherman, "Johann Michael Haydn" in The Symphony: Salzburg, Part 2, London: Garland Publishing (1982): lxiii