Are we to only follow Hadiths from the sihah sittah?
Or are the Hadiths in other books, like Musnad Ahmad, also acceptable?
In matters of jurisprudence (fiqh) we should follow whatever our Fuqaha have deduced from the Hadiths.
One should never endeavour to follow Hadith directly without the guidance of the Fuqaha. See here for more.
However in principle, there are many authentic Hadiths outside the sihah sittah (the six famous books) as well.
This is an undeniable reality.
The Muhaddithun (Hadith experts) throughout time have been declaring Hadiths as authentic, as long they fulfill the criterion for acceptance, irrespective of the book in which the Hadith is found.
Refer to Muqaddimah Ibnus Salah, pg. 32 At-Taqyidu wal Idah and Tadribur Rawi, vol. 2 pg. 372, for a discussion on this particular issue.
And Allah Ta’ala Knows best,
Answered by: Moulana Muhammad Abasoomar