The Messenger of Allah ﷺ asked a man: Did you fast the last day of Sha'ban ? He replied: No. He said: If you did not observe a fast, you must fast for a day. One of the two narrators said: For two days.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ لِرَجُلٍ هَلْ صُمْتَ مِنْ سَرَرِ شَعْبَانَ شَيْئًا قَالَ لاَ قَالَ فَإِذَا أَفْطَرْتَ فَصُمْ يَوْمًا وَقَالَ أَحَدُهُمَا يَوْمَيْنِ  

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.  

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 ﷺ أَمْ شَىْءٌ مِنْ رَأْيِكَ قَالَ سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ صُومُوا الشَّهْرَ وَسِرَّهُ  

I heard Abu 'Amr al-Auza'i say: The word sirrahu means beginning of the month.  

سَمِعْتُ أَبَا عَمْرٍو يَعْنِي الأَوْزَاعِيَّ يَقُولُ سِرُّهُ أَوَّلُهُ  

On the authority of Abu Mushir. He said: Sa'id, that is, Ibn 'Abd al-'Aziz said: The meaning of the word sirraha is "in the beginning of it (the month)"  

وَقَالَ بَعْضُهُمْ سِرُّهُ وَسَطُهُ وَقَالُوا آخِرُهُ