Musnad Ahmad 770:108

Rawh narrated to us, Shu'bah narrated to us, he said: I heard Simak ibn Harb, he said: I heard 'Abdullah ibn 'Umayrah—and he was the guide of Al-A'sha during the Jahiliyyah—narrating from Jarir, he said: I came to the Messenger of Allah, peace and blessings of Allah be upon him, and said: "I pledge allegiance to you upon Islam." He said: He held back his hand and said: "And [to offer] sincere advice to every Muslim." Then he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Indeed, whoever does not show mercy to the people, Allah the Mighty and Majestic will not show mercy to him."