Hadith 13:257
'Imran b. Husain (Allah be pleased with them) reported Allah's Messenger (may peace he upon him) having said to him or to someone else:
Did you fast in the middle of Sha'ban? He said: No. Thereupon he (the Holy Prophet) said: If you did not observe fast, then you should observe fast for two days.
حَدَّثَنَا هَدَّابُ بْنُ خَالِدٍ، حَدَّثَنَا حَمَّادُ بْنُ سَلَمَةَ، عَنْ ثَابِتٍ، عَنْ مُطَرِّفٍ، - وَلَمْ أَفْهَمْ مُطَرِّفًا مِنْ هَدَّابٍ - عَنْ عِمْرَانَ بْنِ حُصَيْنٍ، - رضى الله عنهما - أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ لَهُ أَوْ لآخَرَ " أَصُمْتَ مِنْ سَرَرِ شَعْبَانَ " . قَالَ لاَ . قَالَ " فَإِذَا أَفْطَرْتَ فَصُمْ يَوْمَيْنِ "
Original Reference: Sahih Muslim 1161 b