Muhammad bin Ka'b (may Allah have mercy on him) says: Do not say 'Ramadan', for Ramadan is one of the names of Allah Almighty; rather, say 'the month of Ramadan'.
This has also been narrated from Mujahid and al-Hasan al-Basri, but the chain to them is weak. Muhammad bin Isma'il al-Bukhari, in his Sahih, used the following hadith as evidence that this (saying 'Ramadan') is permissible.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7905