Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says, concerning a man whom Ramadan reaches while he still owes the fasts of another Ramadan: He should fast this Ramadan, feed a poor person for each day of the previous Ramadan, and make it up.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8211