The father of Abu Ishaq says: We were with Sayyiduna Jarir bin Abdullah (may Allah be pleased with him), and we were struck by hunger. So Sayyiduna Jarir bin Abdullah (may Allah be pleased with him) wrote a letter to Sayyiduna Mu'awiyah (may Allah be pleased with him) and wrote in it this saying of the Messenger of Allah (peace and blessings be upon him): "Whoever does not show mercy to people, Allah does not show mercy to him." So Sayyiduna Mu'awiyah (may Allah be pleased with him) wrote that they should return, and Sayyiduna Mu'awiyah (may Allah be pleased with him) gave them provisions. Abu Ishaq says: I came upon a cloak from that.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17903