Mozart and Haydn String Quartets
The composers Wolfgang Amadeus Mozart (1756–1791) and Joseph Haydn (1732–1809) were friends. Their relationship is not very well documented, but the evidence that they enjoyed each other's company is strong. Six string quartets by Mozart are dedicated to Haydn (K. 387, 421, 428, 458, 464, 465, the "Haydn" Quartets).
Haydn was already a fairly well-known composer in Mozart's childhood. His six string quartets Opus 20 (1772), called the "Sun" Quartets from the drawing of the sun on the cover of the first edition, were widely circulated and are conjectured (for instance, by Charles Rosen) to have been the inspiration for the six early string quartets K. 168–173 the 17-year-old Mozart wrote during a 1773 visit to Vienna.
The two composers probably weren't able to meet until after Mozart's permanent relocation to Vienna in 1781. Haydn's presence was required most of the time at the palace of Eszterháza in Hungary some distance from Vienna, where his employer and patron Prince Nikolaus Esterházy preferred to live. During the winter months, the Prince moved to the ancestral palace of his family in Eisenstadt, bringing Haydn with him. In these periods it was often feasible for Haydn to make brief visits to Vienna, about 40 km away.
As Jones notes, there were various points in the 1770s and early 1780s when Haydn and Mozart might have met, Haydn visiting Vienna from his normal work venues of Esterháza and Eisenstadt, Mozart from Salzburg. The earliest at which it is likely they would have met is 22 and 23 December 1783, at a performance sponsored by the Vienna Tonkünstler-Societät, a charitable organization for musicians. On the program were works by both Haydn (Jones: "a symphony and a chorus, both probably from [the oratorio] Il ritorno di Tobia") and Mozart ("a new concert aria, probably 'Misero! o sogno!' [K. 431], and, on the first night, a piano concerto.
Jens Peter Larsen suggests that "quartet playing was central to the contact between Haydn and Mozart", although the documentation of the occasions in which the two composers played or heard quartets or other chamber music together is slim. One report of such an occasion comes from the Reminiscences (1826) of the Irish tenor Michael Kelly, who premiered Mozart's most important operatic lyric tenor roles.
The composer Maximilian Stadler also remembered chamber music performances in which Haydn and Mozart participated: the two of them took the viola parts in performances of Mozart's string quintets, K. 515, 516, and 593.
Mozart's death
Haydn, still in London a year later when the news of Mozart's death reached him, was distraught; he wrote to their mutual friend Michael Puchberg, "For some time I was quite beside myself over his death, and could not believe that Providence should so quickly have called away an irreplaceable man into the next world". Haydn wrote to Constanze Mozart offering musical instruction to her son when he reached the appropriate age, and later followed through on his offer.
Source: https://en.m.wikipedia.org/wiki/Haydn_and_Mozart
Haydn and Mozart - Wikipedia