Al-Musnad of Imam Ahmad

The Musnad of the Imam Ahmad is one of the most important collections of hadith (it is among the Kutub at-Tis'a - the 9 base books in the hadith). It contains nearly 40000 hadiths, which are classed by reporter, ie according to the Sahaba of which the chain goes up. He is known to be a serious collection where most of the rapporteurs are good and accepted as ibn Taymiyya and as-Suyuti says in substance.