Tirmidhi

Commentary: 1 ؎:
This narration does not contradict the previous authentic hadiths.
Imam Bayhaqi, after quoting this narration of Jabir bin Abdullah radi Allahu anhuma in (Kitab al-Qira’ah, p. 112), writes:
In this narration, there is evidence for the specification of Surah al-Fatihah for recitation in prayer, and it is against those scholars who neither accept the specification of al-Fatihah nor consider the recitation of this Surah obligatory in the last two rak‘ahs.
There is also evidence for the obligation of reciting Surah al-Fatihah in every rak‘ah of all units of prayer.
As for the statement of Jabir bin Abdullah radi Allahu anhuma: ((Except behind the Imam)), then:

➊
It is possible that this is his (radi Allahu anhu) position regarding the permissibility of omitting Surah al-Fatihah behind the Imam in those prayers in which the Imam recites Surah al-Fatihah aloud.

➋
It is also possible that:
What is meant is the rak‘ah in which the follower joins the Imam while he is in the bowing (ruku‘) position, and joins him, then that rak‘ah will be counted for him.
This interpretation has been adopted by scholars such as Ishaq bin Ibrahim al-Hanzali.

➌
Ibn Khuzaymah has recorded in his Sahih, from the narration of Yazid al-Faqir, that Jabir bin Abdullah radi Allahu anhuma used to recite Surah al-Fatihah and another Surah in the first two rak‘ahs of a four-rak‘ah prayer, and in the last two rak‘ahs, only Surah al-Fatihah. He said that we used to discuss that prayer is not valid in any rak‘ah without reciting something along with Surah al-Fatihah.
(So, through the action of Jabir radi Allahu anhu, the matter became clear for the people) and that this wording is general:
For the one praying alone, for the follower, and for the Imam as well.

➍ The narration of Ubaydullah bin Muqsim is that Jabir bin Abdullah radi Allahu anhuma used to say:
The Sunnah practice of recitation in prayer is that the one praying should recite Surah al-Fatihah and another Surah in the first rak‘ah, and in the last two rak‘ahs, only Surah al-Fatihah... And the principle is that when a Companion uses the word Sunnah and says: ((We used to discuss)), then the noble hadith scholars count it among the marfu‘ (attributed to the Prophet sallallahu alayhi wa sallam) narrations.
(For further details, see:
Tuhfat al-Ahwadhi 1/261)
Note:
(It should be clear that this is the statement of Jabir radi Allahu anhu,
and in terminology, this is called mawquf.)