Tirmidhi

1:
From the phrase "You also do not say it," it apparently seems that he has altogether prohibited the recitation of ((BISMILLAH)), but it is better to interpret this as a prohibition against reciting it aloud (jahran), because the preceding statement, "I have not heard any of them say it," pertains to reciting aloud, since only that which is said aloud (jahran, with force) can be heard. The author, in the chapter heading (tarjamat al-bab), has also indicated towards this.

2:
Most of the hadiths that have come regarding reciting ((BISMILLAH)) aloud are weak. The best among them is the narration of Nu'aym al-Mujmir, whose wording is as follows: ((I prayed behind Abu Hurayrah radi Allahu anhu, and he recited: BISMILLAH AR-RAHMAN AR-RAHEEM, then he recited Umm al-Qur'an (al-Fatihah) until, when he reached "ghayr al-maghdoobi 'alayhim wa la ad-dalleen," he said: Ameen, and the people said: Ameen)). An objection has been raised against this narration that other people besides Nu'aym al-Mujmir have also narrated from Abu Hurayrah radi Allahu anhu, but in those narrations there is no mention of ((BISMILLAH)). The answer to this is that Nu'aym al-Mujmir is trustworthy (thiqa), and the addition of a trustworthy narrator is accepted; therefore, evidence can be taken from this for reciting aloud (jahr). In summary, both reciting silently (sirran) and aloud (jahran) are permissible, but most and the majority of the authentic hadiths mention reciting it quietly. (See Hadith No: 246)
Note:
(Some imams have declared "Ibn Abdullah ibn Mughaffal" to be unknown (majhul) and thus have considered this narration weak, whereas Hafiz Mizzi has stated his name as "Yazid," and Hafiz Ibn Hajar has declared him "truthful" (saduq).)