Abdur-Rahman bin Abis al-Nakha'i (may Allah have mercy on him) said that Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: That I should not fast one day of Ramadan and then make it up is dearer to me than that I add to it a day that is not part of it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7959