Hadith: Whoever forgets a prayer should perform it as soon as he remembers it; there is no expiation for it except that

عن أنس بن مالك رضي الله عنه عن النبي صلى الله عليه وسلم قال:
«مَنْ نَسِيَ صَلَاةً فَلْيُصَلِّ إِذَا ذَكَرَهَا، لَا كَفَّارَةَ لَهَا إِلَّا ذَلِكَ: {وَأَقِمِ الصَّلاةَ لِذِكْرِي} [طه: 14]».
[صحيح] - [متفق عليه] - [صحيح البخاري: 597]

Anas ibn Mālik (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) said:
"Whoever forgets a prayer should perform it as soon as he remembers it; there is no expiation for it except that: {and establish prayer to remember me.} [Surat Tā-ha: 14]"
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Bukhari - 597]

Explanation

The Prophet (may Allah's peace and blessings be upon him) pointed out that whoever forgets to perform any obligatory prayer until its time has elapsed should hasten to make up for it when he remembers it. The sin of abandoning it cannot be eliminated and covered except by the Muslim's performance upon remembering it. In his noble Book, Allah says: {and establish prayer to remember me.} [Surat Tā-ha: 14], i.e., perform the forgotten prayer when you remember it.