The Messenger of Allah ﷺ never failed to fast on the white days, whether at home or while traveling.
أخبرنا القاسم بن زكريا قال حدثنا عبيد الله قال حدثنا يعقوب عن جعفر عن سعيد
كان رسول الله ﷺ لا يفطر أيام البيض في حضر ولا سفر
Sourcing, its isnad is hasan. It is in Al-Kubra, no. 2654, and Diya Maqdisi included it in Al-Mukhtarah, and Nawawi graded it hasan in Riyad al-Salihin, no. 1265.Ubaydullah is Ibn Musa, Yaqub is Ibn Lahiah, Abdullah is Qummi, Jafar is Ibn Abu Mughira Qummi, and Saeed is Ibn Jubayr.