Al-Humaydī reports, through Sufyān:

“I once saw Jarīr leading Mughīrah by the hand, so I asked ʿUmar ibn Saʿīd, ‘Who is that young man?’ ʿUmar replied, ‘He is a young man of sound character.’

Hanbal related: Abū ʿAbd Allāh (Aḥmad ibn Ḥanbal) was asked, ‘Whom do you prefer, Sharīk or Jarīr?’ He answered, ‘Jarīr makes fewer slips; Sharīk used to make mistakes.’

ʿUthmān ibn Saʿīd said: I asked Yaḥyā, ‘For narrations from Manṣūr, whom do you favor—Jarīr or Sharīk?’ He replied, ‘Jarīr is more knowledgeable about him.’

Aḥmad al-ʿIjlī said: ‘Jarīr was a Kufan, trustworthy, who later settled in Rayy. Whenever someone came to Rabāḥ saying, “I want to record the ḥadīths of Kūfah,” he would tell him, “Take them from Jarīr; and if you miss him, then from Muḥammad ibn Fuḍayl.”’

Ibn Abī Ḥātim said: I asked my father about al-Aḥwaṣ and Jarīr with regard to their narrations from Ḥuṣayn. He replied, ‘Jarīr was the shrewder of the two; Jarīr is dearer to me.’ I asked, ‘Is his ḥadīth used as evidence?’ He said, ‘Yes. Jarīr is reliable, and in narrations from Hishām ibn ʿUrwah he is preferable to me over Yūnus ibn Bukayr.’

Al-Nasāʾī said, ‘Reliable.’
Ibn Khirāsh said, ‘Trustworthy.’
Abū al-Qāsim al-Lālikāʾī said, ‘There is unanimous agreement on his reliability.’

It has been reported that Jarīr himself said, ‘I was born in the year ten’ (110 AH).

Hanbal ibn Isḥāq, however, narrated from Abū ʿAbd Allāh that ‘Jarīr was born in the year one-hundred-and-seven’ (107 AH).

I (the author) add: In that same year, 107 AH, Sufyān ibn ʿUyaynah was born; yet Sufyān began the quest for knowledge earlier than Jarīr, meeting Ziyād ibn ʿIlāqah, ʿAmr ibn Dīnār, and the leading scholars of Kūfah and the Two Sanctuaries.