We sighted the crescent of Ramadan while we were at Dhat Irq. We sent a man to Ibn Abbas to ask him about it.Hashim said, "He asked him, and Ibn Abbas replied, 'The Messenger of Allah ﷺ said, "Allah has extended its sighting,"'" Hashim said, "meaning its sighting, 'so if it is obscured from you, then complete the count.'"
حدثنا محمد بن جعفر وهاشم قالا حدثنا شعبة عن عمرو بن مرة قال سمعت
أهللنا هلال رمضان ونحن بذات عرق قال فأرسلنا رجلا إلى ابن عباس يسأله قال هاشم فسأله فقال ابن عباس قال رسول الله ﷺ إن الله قد مد رؤيته قال هاشم لرؤيته فإن أغمي عليكم فأكملوا العدة
Its isnad is sahih. Abu Bakhtari is Saeed bin Fayruz, a distinguished, trustworthy tabii, and Bukhari stated explicitly in al-Kabir 2/1/464 that he heard from Ibn Abbas and Ibn Umar. The meaning of this hadith has already appeared at greater length in 1985 and 2335.