Hanbal bin Ishaq (may Allah have mercy on him) said: I heard my paternal uncle Abu Abdullah Ahmad bin Hanbal (may Allah have mercy on him) say: The mursal reports of Sa'id bin al-Musayyib (may Allah have mercy on him) are of the rank of sahih. In our view no one's mursal reports are more authentic than his. As for the mursal reports of al-Hasan and Ata' (may Allah have mercy on them both), they are the weakest of all, because they used to take from everyone. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرهن / 11227