Umm Fadl bint Harith sent him to Muawiya in Syria. He said, "I arrived in Syria and took care of her errand. The new moon of Ramadan appeared while I was still in Syria, and we sighted the crescent on the night of Friday. Then I came to Medina at the end of the month, and Abdullah bin Abbas asked me about various matters, then he mentioned the crescent and asked, 'When did you sight it?'I replied, 'We sighted it on the night of Friday.'He asked, 'Did you see it yourself?'I said, 'Yes, and the people saw it and fasted, and Muawiya fasted.'He said, 'But we sighted it on the night of Saturday, so we will continue fasting until we complete thirty days or sight it.'I asked, 'Is the sighting of Muawiya and his fasting not sufficient for you?'He replied, 'No. This is how the Prophet ﷺ commanded us.'"
حدثنا سليمان بن داود الهاشمي حدثنا إسماعيل يعني ابن جعفر قال أخبرني محمد يعني ابن أبي حرملة
أن أم الفضل بنت الحارث بعثته إلى معاوية بالشام قال فقدمت الشام فقضيت حاجتها واستهل علي رمضان وأنا بالشام فرأينا الهلال ليلة الجمعة ثم قدمت المدينة في آخر الشهر فسألني عبد الله بن عباس ثم ذكر الهلال فقال متى رأيتموه فقلت رأيناه ليلة الجمعة فقال أنت رأيته قلت نعم ورآه الناس وصاموا وصام معاوية فقال لكنا رأيناه ليلة السبت فلا نزال نصوم حتى نكمل ثلاثين أو نراه فقلت أولا تكتفي برؤية معاوية وصيامه فقال لا هكذا أمر النبي ﷺ
Its isnad is sahih. Ismail bin Jafar bin Abu Kathir Ansari Qari is trustworthy, reliable, and rarely errs. He shared many of the same teachers with Malik. Bukhari mentioned him in al-Kabir 1/1/349-350.The hadith was narrated by Muslim 1:300, Abu Dawud 2:271, and Tirmidhi 2:350, all of them through the route of Ismail bin Jafar. Tirmidhi said, "Its hadith is hasan sahih gharib." Nasai also narrated it, as mentioned in Awn al-Mabud, quoting from Munziri.