The tribe of Najia was mentioned in the presence of the Messenger of Allah ﷺ, and he said, "They are a tribe descended from me." He did not mention Saad in this report.
حدثنا محمد بن جعفر وذكر الحديث بقصة فيه فقال ابن أخي
قد ذكروا بني ناجية عند رسول الله ﷺ فقال هم حي مني ولم يذكر فيه سعد
Its isnad is daif, for the same reason mentioned in the hadith before it, and also because it is disconnected, since Saad bin Abu Waqqas is not mentioned in it. It is a repetition of the one before it.As for Ahmad's saying, "Muhammad bin Jafar narrated to us," and then he mentions the hadith and so on, he means that Muhammad bin Jafar narrated it to him with the same isnad mentioned before, going back to the nephew of Saad, disconnected, without mentioning Saad in it.The hadith appears in Majma al-Zawaid 10:50, where it says, Ahmad narrated it both connected and disconnected, in abridged form, from the nephew of Saad, without naming him, and the rest of the narrators in both are the narrators of the Sahih. The phrase "from the nephew of the Musannad" as it stands there is an established reading in the Majma, but it is a meaningless error, and I consider it most likely a typesetting mistake.