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).

أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ يَصُومُ حَتَّى يُقَالَ قَدْ صَامَ قَدْ صَامَ وَيُفْطِرُ حَتَّى يُقَالَ قَدْ أَفْطَرَ قَدْ أَفْطَرَ

I asked Sa'id b. Jubair about fasting during Rajab. He said: Ibn 'Abbas told me that the Messenger of Allah ﷺ used to fast to such an extent that we thought that he would never break his fast; and he would go without fasting to such an extent that we thought he would never fast.

أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ يَصُومُ حَتَّى نَقُولَ لاَ يُفْطِرُ وَيُفْطِرُ حَتَّى نَقُولَ لاَ يَصُومُ

I asked Sa'id b. Jubair about fasting during Rajab. He said: Ibn 'Abbas told me that the Messenger of Allah ﷺ used to fast to such an extent that we thought that he would never break his fast; and he would go without fasting to such an extent that we thought he would never fast. (Using translation from Abū Dāʾūd 2430)

أَنَّ رَسُولَ اللهِ ﷺ كَانَ يَصُومُ حَتَّى نَقُولَ لَا يُفْطِرُ وَيُفْطِرُ حَتَّى نَقُولَ لَا يَصُومُ

[Machine] The Prophet ﷺ used to fast until it would be said, "He had kept fasting" and would break his fast until it would be said, "He had broken his fast."

أَنَّ النَّبِيَّ ﷺ كَانَ يَصُومُ حَتَّى يُقَالَ صَامَ صَامَ وَيُفْطِرُ حَتَّى يُقَالَ أَفْطَرَ أَفْطَرَ

[Machine] The Prophet Muhammad ﷺ used to fast until it was said that he had fasted, and he would break his fast until it was said that he had broken his fast. He once said "Fast, fast, fast."

أَنَّ رَسُولَ اللهِ ﷺ كَانَ يَصُومُ حَتَّى يُقَالَ قَدْ صَامَ وَيُفْطِرُ حَتَّى يُقَالَ قَدْ أَفْطَرَ وَقَدْ قَالَ مَرَّةً أَفْطَرَ أَفْطَرَ أَفْطَرَ