Sayyiduna Anas (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "If someone forgets a prayer, let him pray it when he remembers it; there is no expiation for it other than that." Allah Almighty says: "Establish the prayer for My remembrance." [Surah Ta-Ha: 14]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4388