Sahih Muslim 1158
Sahih Muslim · Book 13: The Book of Fasting · The Prophet's Fasts at times other than Ramadan; and it is recommended to ensure that no month is free of fasting
وَحَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ ، وَابْنُ أَبِي خَلَفٍ ، قَالاَ حَدَّثَنَا رَوْحُ بْنُ عُبَادَةَ ، حَدَّثَنَا حَمَّادٌ ، عَنْ ثَابِتٍ ، عَنْ أَنَسٍ ، رضى الله عنه ح. وَحَدَّثَنِي أَبُو بَكْرِ بْنُ نَافِعٍ ، - وَاللَّفْظُ لَهُ - حَدَّثَنَا بَهْزٌ ، حَدَّثَنَا حَمَّادٌ ، حَدَّثَنَا ثَابِتٌ ، عَنْ أَنَسٍ ، رضى الله عنه أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ يَصُومُ حَتَّى يُقَالَ قَدْ صَامَ قَدْ صَامَ . وَيُفْطِرُ حَتَّى يُقَالَ قَدْ أَفْطَرَ قَدْ أَفْطَرَ .
Anas (Allah be pleased with him) reported that the Messenger of Allah (ﷺ) used to observe fast till it was said that he had observed fast, he had observed fast (perhaps never to break it), and he did not fast till it was said that he had given up fast, he had given up fast (perhaps never to observe it).