It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: "Whoever, without a valid excuse for which Allah Almighty has granted permission (such as travel or illness), misses a single fast of Ramadan, then fasting for the whole lifetime will not make up for it."
That is, even if he were to fast until the Day of Judgment, he would not attain the reward that is obtained from a single fast of Ramadan; however, some scholars have said: This was stated as an exaggeration, otherwise two consecutive months of fasting are sufficient as a substitute.