Ibrahim said, "I know it only from the Prophet ﷺ." Ubayy said that Yaqub did not express any doubt. He said, "The merit of praying in congregation over one of you praying alone is twenty-five parts."
حدثنا أبو كامل حدثنا إبراهيم عن ابن شهاب عن سعيد بن المسيب
إبراهيم لا أعلمه إلا عن النبي ﷺ قال أبي ولم يشك يعقوب قال فضل صلاة الجماعة على صلاة أحدكم وحده خمسة وعشرين جزءا
Its chain of transmission is sahih. Ibn Majah 787 also related it from Abu Marwan Uthmani, from Ibrahim bin Saad, with this same chain and wording, in abridged form. Its meaning has already passed within a longer hadith, 7185, through the narration of Maamar, from Zuhri. Something similar in meaning has also passed within another hadith by another route, 7424.As for his statement "the twenty-five," this is how it is established in the three source manuscripts here. Abu Kamil's doubt about whether it is marfu, in his narration from Ibrahim bin Saad, despite his preference for the marfu ruling, does not affect this, since Yaqub bin Ibrahim related it from his father as marfu without any doubt, as Imam Ahmad indicated following the chain, and since Abu Marwan Uthmani related it from Ibrahim as marfu without doubt, as found in Ibn Majah, and since the hadith is established as marfu through many routes.