Bakr bin Abdullah (may Allah have mercy on him) said that Anas bin Malik (may Allah be pleased with him) saw no harm in it, and he used to say that Allah Almighty has said: "Then a number of other days" [Al-Baqarah: 184], "complete this number on other days."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8240