As for Umar, it is possible that he took this position before hearing the account of Tamīm, and that once he heard it, he did not repeat the aforementioned oath.

As for Jābir, he witnessed Umar swear that oath in the presence of the Prophet ﷺ, and he continued to uphold what he had learned from Umar in the Prophet’s presence.” (1)

I say: Yet Jābir—may Allah be pleased with him—was himself among the narrators of the hadith of Tamīm, as related in Abū Dāwūd’s narration. After recounting the story of al-Jassāsa and the Dajjal in terms similar to Tamīm’s account, Ibn Abī Salama (2) said: “There is part of this hadith that I did not retain. Abū Salama said (3): ‘Jābir testified that he was Ibn Sā’id.’ I said, ‘But he died.’ He replied, ‘Even if he died.’ I said, ‘But he embraced Islam.’ He replied, ‘Even if he embraced Islam.’ I said, ‘But he entered Medina.’ He replied, ‘Even if he entered Medina.’” (4)

Thus, Jābir—may Allah be pleased with him—remained adamant that Ibn Sayyad was the Dajjal, even if it is said: