Musnad Ahmad 770:10

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Simak ibn Harb, from ‘Ubayd Allah ibn Jarir, from Jarir, who said: I came to the Messenger of Allah, may Allah bless him and grant him peace, and said: "I pledge allegiance to you upon Islam." He held back his hand and said: "Sincerity toward every Muslim." Then the Messenger of Allah, may Allah bless him and grant him peace, said: "Indeed, whoever does not show mercy to the people, Allah the Almighty and Majestic will not show mercy to him."