Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: Whoever leaves one fast of Ramadan without an excuse, the fasts of his whole life will not benefit him, until he meets Allah Almighty; if He wills He will forgive him, and if He wills He will punish him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8066