Hadith 4:157
Abu Usama narrated to us, he said: Ubaydullah ibn Umar narrated to us, from Nafi’, from Ibn Umar, that the Messenger of Allah (may Allah bless him and grant him peace) mentioned Ramadan and said: "Fast at its sighting and break your fast at its sighting. But if it is obscured from you, then estimate it."
حدثنا أبو أسامة قال حدثنا عبيد الله بن عمر عن نافع عن بن عمر ان رسول الله صلى الله عليه وسلم ذكر رمضان فقال صوموا لرؤيته وافطروا لرؤيته فان غم عليكم فاقدروا له