Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) transmits this saying of the Prophet (peace and blessings be upon him): "Indeed, whoever does not show mercy to people, Allah too does not show mercy to him."
Ahmad recorded it; its chain contains a narrator, Atiyyah, that is al-Awfi, who is da'if (weak), though he has been declared trustworthy; its remaining narrators are the narrators of the Sahih.