By Shaykh 'Abdul-'Azeez ar-Raajihee
The four sunan (Aboo Daawood, At-Tirmidhee, An-Nasaa’ee, and Ibn Majah), the musnad of Imaam Ahmad, and the sunan of Ad-Daarimee, all contain ahaadeeth that are saheeh (authentic), hasan (good), and da’eef (weak). For they did not make it a condition for their books to contain only the ahaadeeth that are saheeh as Al-Bukhaaree and Muslim did. Rather, their intent was to gather all the ahaadeeth as they relate to a particular matter. Similarly are those other such the musnad of At-Taylasee and the three ma’ajim of At-Tabaraanee. As for the muwatta of Imaam Maalik, it contains ahaadeeth that are saheeh, hasan, and da’eef.

Al-Haafidh Ibn Katheer, Ibn Qayyim and other than them sometimes mentioned ahaadeeth without paying much attention to their authenticity, and perhaps this is because of either two reasons: 1. There are other ahaadeeth that strengthens that hadeeth or 2. It is a hadeeth that encourages the good or admonishes against the evil, and therefore there is certain amount of leeway that can be taken with regards to them. 

The manaahij (methodologies) of the muhaditheen  (scholars of hadeeth)

The four sunan (Aboo Daawood, At-Tirmidhee, An-Nasaa’ee, and Ibn Majah), the musnad of Imaam Ahmad, and the sunan of Ad-Daarimee, all contain ahaadeeth that are saheeh (authentic), hasan (good), and da’eef (weak). For they did not make it a condition for their books to contain only the ahaadeeth that are saheeh as Al-Bukhaaree and Muslim did. Rather, their intent was to gather all the ahaadeeth as they relate to a particular matter. Similarly are those other such the musnad of At-Taylasee and the three ma’ajim of At-Tabaraanee. As for the muwatta of Imaam Maalik, it contains ahaadeeth that are saheeh, hasan, and da’eef