Sunan an-Nasa'i, Hadith 2359 ⓘ
It was narrated from Usamah bin Zaid: That the Messenger of Allah used to fast continually and it ws said that he would not break his fast, and he used not to fast and it was said that he would not fast.
أَخْبَرَنَا أَحْمَدُ بْنُ سُلَيْمَانَ، قَالَ حَدَّثَنَا زَيْدُ بْنُ الْحُبَابِ، قَالَ أَخْبَرَنِي ثَابِتُ بْنُ قَيْسٍ الْغِفَارِيُّ، قَالَ حَدَّثَنِي أَبُو سَعِيدٍ الْمَقْبُرِيُّ، قَالَ حَدَّثَنِي أَبُو هُرَيْرَةَ، عَنْ أُسَامَةَ بْنِ زَيْدٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ يَسْرُدُ الصَّوْمَ فَيُقَالُ لاَ يُفْطِرُ وَيُفْطِرُ فَيُقَالُ لاَ يَصُومُ .
also referenced under: Book 22, Hadith 270 (In-Book) & Volume 3, Book 22, Hadith 2361 (DarusSalam)