Claim: Mozart's friendship with Haydn can be seen in the Haydn quartets he published in 1785, pieces 387, 421, 428, 458, 464, and 465, which were dedicated to Haydn at a time where most pieces were dedicated to aristocrats.
Evidence: During this time, Haydn and Mozart had become friends, and sometimes played quartets together in Mozart's apartment, with Mozart playing the viola, and Haydn playing violin; see Haydn and Mozart. The "Haydn" Quartets by Wolfgang Amadeus Mozart are a set of six string quartets published in 1785 in Vienna as his Op. 10, dedicated to the composer Joseph Haydn. String Quartet No. 14 in G major, ("Spring"), K. 387, Op. 10, No. 1 (31 December 1782) String Quartet No. 15 in D minor, K. 421/417b, Op. 10, No. 2 (17 June 1783) String Quartet No. 17 in B-flat major ("Hunt"), K. 458, Op. 10, No. 3 (9 November 1784) String Quartet No. 16 in E-flat major, K. 428/421b, Op. 10, No. 4 (June–July 1783) String Quartet No. 18 in A major, K. 464, Op. 10, No. 5 (10 January 1785) String Quartet No. 19 in C major ("Dissonance"), K. 465, Op. 10, No. 6 (14 January 1785)