Narrated Mu'awiyah: AbulAzhar al-Mughirah ibn Farwah said: Mu'awiyah stood among the people at Dayr Mustahill lying at the gate of Hims. He said: O people, we sighted the moon on such-and-such day. We shall fast in advance. Anyone who likes to do so may do it. Malik ibn Hubayrah as-Saba'i stood up and asked: Mu'awiyah, did you hear the Messenger of Allah (ﷺ) say something (about this matter), or is this something on the basis of your opinion? He replied: I heard the Messenger of Allah (ﷺ) as saying: Fast the month (in the beginning) and in the last.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا إِبْرَاهِيمُ بْنُ الْعَلاَءِ الزُّبَيْدِيُّ، مِنْ كِتَابِهِ حَدَّثَنَا الْوَلِيدُ بْنُ مُسْلِمٍ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ الْعَلاَءِ، عَنْ أَبِي الأَزْهَرِ الْمُغِيرَةِ بْنِ فَرْوَةَ، قَالَ قَامَ مُعَاوِيَةُ فِي النَّاسِ بِدَيْرِ مِسْحَلٍ الَّذِي عَلَى بَابِ حِمْصَ فَقَالَ يَا أَيُّهَا النَّاسُ إِنَّا قَدْ رَأَيْنَا الْهِلاَلَ يَوْمَ كَذَا وَكَذَا وَأَنَا مُتَقَدِّمٌ بِالصِّيَامِ فَمَنْ أَحَبَّ أَنْ يَفْعَلَهُ فَلْيَفْعَلْهُ . قَالَ فَقَامَ إِلَيْهِ مَالِكُ بْنُ هُبَيْرَةَ السَّبَئِيُّ فَقَالَ يَا مُعَاوِيَةُ أَشَىْءٌ سَمِعْتَهُ مِنْ رَسُولِ اللَّهِ صلى الله عليه وسلم أَمْ شَىْءٌ مِنْ رَأْيِكَ قَالَ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ " صُومُوا الشَّهْرَ وَسِرَّهُ " .
sunnah.com