Musnad Ahmad 770:105

'Abd al-Rahman—who is Ibn Mahdi—narrated to us, Sufyan narrated to us, from Ziyad ibn 'Alaqah, who said: I heard Jarir ibn 'Abd Allah on the pulpit saying: "I pledged allegiance to the Messenger of Allah, may Allah bless him and grant him peace, and he stipulated upon me sincerity toward every Muslim." So, I am indeed a sincere advisor to you.