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).
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 .
Muslim
The Book of Fasting • Hadith 234