Hadith 1975

He reported God’s messenger as saying, "Calculate on the basis of the new moon of Sha'ban when Ramadan begins." Tirmidhi transmitted it.

وَعَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «أحصوا هِلَال شعْبَان لرمضان» . رَوَاهُ التِّرْمِذِيّ

Reference: 5619750
In-book reference: Book 7, Hadith 1975