Iyas bin Mu'awiyah bin Qurrah al-Muzani (may Allah have mercy on him) narrates: We were with Umar bin Abd al-Aziz (may Allah have mercy on him), and modesty was mentioned in his presence, so the people said: "Modesty is part of the religion." Umar bin Abd al-Aziz said: "Rather, it is the whole of the religion." Iyas (may Allah have mercy on him) said: My father narrated to me from my grandfather Qurrah (may Allah be pleased with him) that he said: We were with the Prophet (peace and blessings be upon him), and modesty was mentioned in his (peace and blessings be upon him) presence, so they said: "O Messenger of Allah (peace and blessings be upon him)! Is modesty part of the religion?" The Messenger of Allah (peace and blessings be upon him) said: "Rather, it is the whole of the religion." Then the Messenger of Allah (peace and blessings be upon him) said: "Modesty is chastity; and inability is inability of the tongue, not of the heart; and action is part of faith. These bring increase in the Hereafter and decrease in this world, and what they add in the Hereafter is greater than what they add in this world."
Iyas bin Mu'awiyah (may Allah have mercy on him) said: Umar bin Abd al-Aziz (may Allah have mercy on him) ordered me, so I dictated it to him; then he wrote it down in his own hand. Then he led us in the Zuhr and Asr prayers while it (the writing) was in his sleeve, and he was greatly pleased with it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20808