Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrated that the Messenger of Allah (peace and blessings be upon him) came to Sayyiduna Umar (may Allah be pleased with him), and some Companions were with him. He (peace and blessings be upon him) said: "Are you believers?" Though he asked three times, they all remained silent. At the end Sayyiduna Umar (may Allah be pleased with him) said: Yes, O Messenger of Allah. He (peace and blessings be upon him) said: "On what basis?" Sayyiduna Umar (may Allah be pleased with him) said: We hope for reward from Allah. The Messenger of Allah (peace and blessings be upon him) said: "By the Lord of the Ka'bah, you are believers."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11288