Abd al-Rahman bin al-Miswar bin Makhramah (may Allah have mercy on him) said: I set out with my father, Sa'd bin Abi Waqqas (may Allah be pleased with him) and Abd al-Rahman bin al-Aswad bin Abd Yaghuth al-Zuhri in the year of Adhruh, and the plague broke out in Syria. We stayed at a place called Sargh for fifty nights. Ramadan began, and al-Miswar (may Allah be pleased with him) and Abd al-Rahman bin al-Aswad fasted, while Sa'd bin Abi Waqqas (may Allah be pleased with him) did not fast and refused to fast. I said to Sa'd (may Allah be pleased with him): "O Abu Ishaq! You are a Companion of the Prophet (peace and blessings be upon him) and took part in Badr; al-Miswar and Abd al-Rahman are fasting and you have not fasted?" Sa'd (may Allah be pleased with him) said: "I have more understanding (of the religion) than they do."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5481