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Anas said, “God's Messenger used to break his fast during a month so that we imagined he had not fasted during it at all, and he fasted so that we imagined he had not broken it. You did not wish to see him praying during the night without doing so, or sleeping without doing so.” Bukhari transmitted it.
Source: sunnah.com
Other translations
Arabic
عَنْ أَنَسٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُفْطِرُ مِنَ الشَّهْرِ حَتَّى يُظَنَّ أَنْ لَا يَصُومَ مِنْهُ وَيَصُومُ حَتَّى يُظَنَّ أَنْ لَا يُفْطِرَ مِنْهُ شَيْئًا وَكَانَ لَا تَشَاءُ أَنْ تَرَاهُ مِنَ اللَّيْلِ مُصَلِّيًا إِلَّا رَأَيْتَهُ وَلَا نَائِمًا إِلَّا رَأَيْتَهُ. رَوَاهُ البُخَارِيّ
sunnah.com