It was recorded by al-Nasa’i (8403), al-Hakim (3/437), Ibn Abi Shaybah (14/462/1875), Ibn Abi ‘Asim in al-Sunnah (2/608), and al-Bazzar in his Musnad (2/338/1814), through various chains from ‘Awf, from him. Ibn Abi Shaybah and al-Bazzar added—but mentioned only Umar’s dispatch:

“He met the people of Khaybar, but they repelled and routed him and his companions. They returned to the Messenger of Allah, peace and blessings be upon him, with him accusing his companions of cowardice and his companions accusing him of cowardice.”

Perhaps this is one of the objectionable reports of this Maimun, the freedman of ‘Abd al-Rahman ibn Samurah, for they were unanimous in declaring him weak. Ibn Hibban differed, listing him in al-Thiqat (5/418), yet even he said:

“Yahya al-Qattan held a poor opinion of him.”

Al-Dhahabi mentioned him in al-Mughni and said:

“Ahmad said: ‘His hadiths are objectionable.’”

He was also corroborated by al-Musayyib ibn Muslim al-Azdi, who said: ‘Abd Allah ibn Buraydah narrated it to us similarly. It includes mention of Abu Bakr and Umar, and that they fought fiercely but without achieving victory; however, it contains the statement about accusing one another of cowardice.

Al-Hakim recorded it in abridged form (3/37), and al-Bayhaqi in full (4/210–212). Al-Hakim said:

“The chain of transmission is authentic.”

Al-Dhahabi agreed with him!

I say: I have not found a biographical notice for this al-Musayyib in the sources available to me. Al-Mizzi neither mentioned him among those who narrated from ‘Abd Allah ibn Buraydah nor among the shaykhs of Yunus ibn Bukayr, who narrates from him here. Thus, he apparently is unknown. And Allah knows best.