It took Brahms more than 20 years to write his first symphony. The second premiered 1 year after the first; the third came six years later, and his fourth and last symphony after another 2 years. All 4 symphonies are remarkably different from one another, and yet all of them are quintessentially Brahms.
In case you don’t have it at hand, here’s a quick link to the score.
The first thematic idea of the exposition is characterized by a dreamy cantability; the violins play the theme in octaves, softly supported by the held sounds of the horns and the arpeggios of the violas and cellos. The woodwinds, minus the oboes, counterpoint with the same theme in thirds, 2 quarter notes apart from the violins.