Muwatta Malik, Book 18, Hadith 37 (USC-MSA) ⓘ

Yahya related to me from Malik that he used to hear the people of knowledge say,"There is no harm in fasting continuously as long as one breaks the fast on the days on which the Messenger of Allah, may Allah bless him and grant him peace, forbade fasting, namely, the days of Mina, the day of Adha and the day of Fitr, according to what we have heard." Malik said, "This is what I like most out of what I have heard about the matter."

وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ سَمِعَ أَهْلَ الْعِلْمِ، يَقُولُونَ لاَ بَأْسَ بِصِيَامِ الدَّهْرِ إِذَا أَفْطَرَ الأَيَّامَ الَّتِي نَهَى رَسُولُ اللَّهِ صلى الله عليه وسلم عَنْ صِيَامِهَا وَهِيَ أَيَّامُ مِنًى وَيَوْمُ الأَضْحَى وَيَوْمُ الْفِطْرِ فِيمَا بَلَغَنَا ‏.‏ قَالَ وَذَلِكَ أَحَبُّ مَا سَمِعْتُ إِلَىَّ فِي ذَلِكَ ‏.‏