Is A Mursal Hadeeth Weak?
A mursal narration is a narration that a taabi’ee ascribes to the Prophet (saw) without mentioning the Companion that he took it from.
“This is a vast science because the Imaams differ regarding the validity of narrations that do not have complete chains.” 
– The fuqahaa (scholars of fiqh) disagree as to whether the mursal hadeeth is a valid proof (most of them saying it is weak – see An-Nawawee’s quote below).
– The hadeeth scholars, however, agree among themselves that it is not a valid proof.
Imam Muslim said in the introduction to his saheeh:
“In our basic view and the view of the scholars of narrations, the mursal hadeeth is not a valid proof, neither in our view nor in the view of the learned scholars of hadeeth.” [Muqaddimah, Baab Sihhat Al-Ihtijaaj Bil-Hadeeth Al-Mu’an’an]
Al-Haafidh Ibn As-Salaah said:
“As to our mention of the invalidity of the mursal hadeeth and our ruling of it being weak, the group of hadeeth retainers and critics of narrations agree with our view. A view they expressed in their various works.” [See Uloom Al-Hadeeth of Ibn As-Salah, p.55]
An-Nawawee has been quoted to have said:
“The mursal hadeeth is a weak hadeeth according to the vast majority of hadeeth scholars, Ash-Shaafi’ee, and most of the fuqahaa (scholars of fiqh) and jurisprudents of usool.” [See Tadreeb of As-Suyootee, p.119]
This is because of ignorance of the condition of the missing link which may possibly be weak who may be a person other than a Companion.