Definition of a mursal hadeeth
Question

                What is a mursal hadeeth?.
Praise be to Allah.
    A hadeeth is called a “mursal hadeeth” if the one who 
    narrated it from the Prophet (blessings and peace of Allah be upon him) is a 
    Taabi’i and not a Sahaabi. It is possible to differentiate between a Sahaabi 
    and a Taabi’i by referring to books of biography. 

    Imam Abu ‘Abd-Allah al-Haakim (may Allah have mercy on him) 
    said: 

    There is no difference of opinion among the shaykhs of 
    hadeeth that the mursal hadeeth is one which the muhaddith narrated with a 
    complete isnaad back to the Taabi’i, and the Taabi’i says, “The Messenger of 
    Allah (blessings and peace of Allah be upon him) said.” End quote. 

    Ma’rifah ‘Uloom al-Hadeeth, 67 

    Ibn ‘Abd al-Barr (may Allah have mercy on him) said: 

    This name is given by consensus to the hadeeth of a senior 
    Taabi’i from the Messenger of Allah (blessings and peace of Allah be upon 
    him), such as when ‘Ubayd-Allah ibn ‘Adiyy ibn al-Khayyaar, or Abu Umaamah 
    ibn Sahl ibn Haneef, or ‘Abd-Allaah ibn ‘Aamir ibn Rabee’ah, and the like, 
    say: The Messenger of Allah (blessings and peace of Allah be upon him) 
    said… 

    The same applies to those who are lower in status than these, 
    such as Sa’eed ibn al-Musayyab, Saalim ibn ‘Abd-Allaah, Abu Salamah ibn ‘Abd 
    al-Rahmaan, al-Qaasim ibn Muhammad, and others like them. 

    The same applies to ‘Ilqimah ibn Qays, Masrooq ibn al-Ajda’, 
    al-Hasan, Ibn Sireen, al-Sha’bi, Sa’eed ibn Jubayr, and others like them 
    among the rest of the Taabi’een who it has been proven met a number of the 
    Sahaabah and spent time with them. This is what is meant by mursal according 
    to the scholars. 

    It also applies to reports narrated from Taabi’een of lower 
    status than those mentioned above, such as hadeeth narrated by Ibn Shihaab, 
    Qataadah, Abu Haazim and Yahya ibn Sa’eed from the Prophet (blessings and 
    peace of Allah be upon him), which they call mursal, like the mursal reports 
    of the senior Taabi’een. End quote. 

    It is essential to point out that the muhadditheen -- 
    especially the earlier ones among them -- called every report that had an 
    interruption in its isnaad a mursal report. 

    Al-Khateeb al-Baghdadi (may Allah have mercy on him) said: 

    Mursal refers to a report whose isnaad (chain of 
    transmission) is interrupted, meaning that among its narrators is one who 
    did not hear it from the one whose name comes before his. But in most cases, 
    what is described as mursal is that which was narrated by the Taabi’i from 
    the Prophet (blessings and peace of Allah be upon him). End quote. 

    Al-Kifaayah, p. 21.