The Messenger of Allah ﷺ said to me, "Fast three days from every month and recite the Quran once a month."I asked him to lessen it, and he kept lessening it for me, until he said, "Fast one day and break your fast the next."Ata said, "We differed about what Ubayy reported. Some of us said seven days, and some of us said five."
حدثنا سليمان بن حرب أخبرنا حماد عن عطاء بن السائب عن أبيه
قال لي رسول الله ﷺ صم من كل شهر ثلاثة أيام واقرأ القرآن في شهر فناقصني وناقصته فقال صم يوما وأفطر يوما قال عطاء واختلفنا عن أبي فقال بعضنا سبعة أيام وقال بعضنا خمسا
No references are available.
[Its isnad is hasan] Ahmad narrated it: 2/162, 216, from the hadith of Ata bin Saaib, transmitted through him. Hammad is Ibn Zayd.