Mishkat al-Masabih #1884
Fasting
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وَعَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «أحصوا هِلَال شعْبَان لرمضان» . رَوَاهُ التِّرْمِذِيّ
He reported God’s messenger as saying, "Calculate on the basis of the new moon of Sha'ban when Ramadan begins."
Tirmidhi transmitted it.
About Mishkat al-Masabih
This hadith is #1884 from Mishkat al-Masabih, in the Book of Fasting.