All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.
Imaam Ahmad presented this Hadeeth as follows: ‘ʻ‘Abd Ar-Razzaaq related to us that Al-Munthir ibn An-Nuʻmaan Al-Aftas said: I heard Wahb relate that Ibn ʻAbbaas said that the Prophet said: "An army of twelve-thousand men will come out of Aden-Abyan. They will aid Allaah and His Messenger; they are the best between myself and them." One of the transmitters (ʻAbd Ar-Razzaaq) said, "Maʻmar said to me: 'Go and ask him about this Hadeeth.'"”
Ibn Qudaamah cited it in his Al-Muntakhab min ʻIlal Al-Khallaal with a chain from the route of Imaam Ahmad and he attributed the following statement to Ahmad: “Al-Munthir ibn An-Nu‘maan is a trustworthy reporter from Sanʻaa’. This is the only Hadeeth he attributed to the Prophet with a continuous chain.”
This serves as a declaration of the trustworthiness of Al-Munthir. The rest of the narrators in the chain are also trustworthy and were accepted by Al-Bukhari and Muslim. Al-Haythami said, “Abu Yaʻla and At-Tabaraani narrated it … The reporters in the chain of narration were accepted by Al-Bukhari and Muslim except for Munthir Al-Aftas, who is however a trustworthy narrator.” [Majmaʻ Az-Zawaa’id]
Al-Booseeri said, “Abu Yaʻla Al-Mawsili and Ahmad ibn Hanbal presented it with a chain of narration consisting of trustworthy reporters.” [It-haaf Al-Khiyarati Al-Maharah]
Ahmad Shaakir said, “Its chain of narration is authentic. Al-Munthir ibn An-Nuʻmaan Al-Aftas the Yemeni was declared a trustworthy reporter by Ibn Ma‘een. Ibn Hibbaan also listed him among the trustworthy narrators, and Al-Bukhari included his biography in his book (At-Taareekh) Al-Kabeer. The fact that Maʻmar asked ʻAbd Ar-Razzaaq to go and listen to the Hadeeth from him confirms his trustworthiness.” [Musnad, edited by Shaakir]
Al-Albaani declared it Saheeh (sound) in his book As-Silsilah As-Saheehah.
Allaah Knows best.