The view of Imam Ahmad regarding what may be accepted as evidence is based on five things:
1.. The texts of the Qur’an and saheeh Sunnah.
2.. The fatwas of the Sahaabah: if one of them issued a fatwa and there is no difference of opinion known among the Sahaabah regarding it, then he would not ignore it and move on to something else.
3.. If the Sahaabah disagreed concerning the matter, then he would choose from among their views that which is closest to the Qur’an and Sunnah, and he would not choose any view other than theirs. If it was not certain which view was closest to the Qur’an and Sunnah, he would list the different views of the Sahaabah without stating which is more likely to be correct.
4.. Accepting mursal and da‘eef hadiths as evidence.
5.. Analogy (qiyaas).
What is meant by da‘eef (weak) hadiths is those that come under the heading of hasan, or those that are only slightly weak, not those that are munkar (odd) or baatil (false), or those of which the isnaad included any narrator who was accused of lying.
Accepting this type (of da‘eef hadith) and mursal hadiths as evidence is the view of most of the fuqaha’; it is not something that is unique to Ahmad.
Ibn al-Qayyim (may Allah have mercy on him) said: The fourth principle: accepting mursal hadiths and da‘eef hadiths as evidence, if there is no other text concerning the matter that would contradict them. This type of hadith is given precedence over analogy (qiyaas).
What is meant by da‘eef, in his view, is not that which is baatil (false) or munkar (odd), and not that of which any of the narrators was accused of lying, so that the report is rendered unacceptable and cannot be used as evidence. Rather the da‘eef hadith, in his view, is a counterpart of that which is saheeh, and that which is hasan comes under the heading of that which is da‘eef. He did not divide hadiths into [the three categories of] saheeh, hasan and da‘eef. In his view, the da‘eef hadiths are of various levels, so if he could not find a sound report to contradict the da‘eef hadith, or the view of a Sahaabi, or consensus that was contrary to the da‘eef hadith, it was more appropriate to refer to this da‘eef hadith and use it as evidence, rather than turning to analogy.
All the imams agreed with him on this principle in general, because all of them gave precedence to da‘eef hadiths over analogy.
Abu Haneefah gave precedence to the hadith about laughing in prayer over mere analogy, even though the scholars of hadith are unanimously agreed that it is da‘eef. He gave precedence to the hadith about doing wudoo’ using nabeedh of dates [in the absence of water] over analogy, even though most of the scholars of hadith are of the view that it is da‘eef. He gave precedence to the hadith, “The maximum duration of menses is ten days” – which is da‘eef according to their consensus – over mere analogy, even though the blood on the thirteenth day may be like that on the tenth day in quantity, nature and description. He gave precedence to the hadith, “There should be no mahr less than ten dirhams” – even though they are agreed that it is da‘eef and even baatil – over mere analogy, even though the mahr is given in return for intimacy, so whatever the couple both agree to should be permissible, whether the amount is small or great.
Ash-Shaafa‘i give precedence to the report about the prohibition on hunting in Wajj (a valley in at-Taa’if), even though the report is da‘eef, over analogy. He gave precedence to the report about the permissibility of praying in Makkah at times when prayer is ordinarily prohibited, even though it is da‘eef and goes against the analogy comparing Makkah with other lands. According to one of the two views narrated from him, he gave precedence to the hadith, “Whoever vomits or has a nosebleed [whilst praying], let him do wudoo’ and resume his prayer [from where he left off]” over analogy, even though the report is da‘eef and mursal.
As for Maalik, he gave precedence to mursal hadiths, munqati‘ hadiths and the views of the Sahaabah over analogy."(I‘laam al-Muwaqqi‘een 1/25).
Based on that:
It is not valid to say that the Hanbali madhhab is quite lax and easy-going in this regard, which may give the false impression that other madhhabs do not accept hadiths that are mursal, hasan or slightly da‘eef. Rather all the madhhabs do that, and whoever is familiar with the books of the fuqaha’ will not doubt that.
And Allah knows best.