Q: If a person missed a the scheduled prayer. Is it necessary to still pray it at another time. For example if I missed the fajr prayer can I still pray it later on?

A: Praise be to Allah(swt).  The answer to this question may be found in the following Hadith:

Anas ibn Maalik said: “The Messenger of Allah(saw) said: ‘If one of you sleeps and misses a prayer, or forgets it, then let him pray it as soon as he remembers, because Allah(swt) says (interpretation of the meaning): ‘… and perform al-salaah for My remembrance.’ [Ta-Ha 20:14]” (Reported by Muslim).

Abu Hurayrah reported that the Messenger of Allah(saw) said: “Whoever forgets the prayer, let him pray as soon as he remembers, because Allah(swt) says (interpretation of the meaning): ‘… and perform al-salaah for My remembrance.’ [Ta-Ha 20:14]” (Reported by Muslim).

Anas ibn Maalik reported that the Messenger of Allah(saw) said: “Whoever forgets a prayer, let him pray it as soon as he remembers, for there is no expiation for it except this.” (Reported by Muslim, 1102).

And Allah(swt) knows best. 

Repentance for neglecting prayer

Q: I am 38 years old man. Till the age of 25 I didn't pray. Please advice me what I have to do for this? 

A: Praise be to Allah(swt).
Remember how Allah(swt) has blessed you by bringing you back to Islam after you have been neglecting salaah, and keep on doing the prayers at the right times. Do more naafil (supererogatory or extra) prayers to
make up for the fard (obligatory) prayers that you have missed, as was advised in the saheeh hadith narrated by Hurayth ibn Qubaysah, who said: “I came to Madeenah and prayed, ‘O Allah(swt), send me a righteous companion.’ I went and sat with Abu Hurayrah and said: ‘I asked  Allah(swt) to bless me with a righteous companion; tell me a hadith that you heard from the Messenger of Allah(saw), so that I may benefit therefrom.’ He said: ‘I heard the Messenger of Allah(saw) say that the first thing for which a person will be brought to account on the Day of Resurrection will be his prayer. If it is sound, he will be successful, and if it is lacking in
any way, he will be doomed. If his obligatory prayers are lacking, the Lord will say: “Look and see whether My slave has any voluntary prayers which may be used to make up what is lacking in his obligatory prayers.” Then all his deeds will be examined and dealt with in the same way.’” (Reported by al-Tirmidhi).

Abu Dawood reported from Anas ibn Hakeem al-Dabbi that he came to Madeenah and met Abu Hurayrah. He said: “He asked me about my lineage so I told him about it, then he said: ‘O young man, shall I not tell you a hadith?’ I said, ‘Of course, may Allah(swt) have mercy on you,’ and I think he mentioned it from the Prophet(saw). He said: ‘The first thing that people will be asked about on the Day of Resurrection will be their prayers. Our Lord will say to His angels (although He knows best): “Look at My slave’s prayers: are they
complete or is something lacking?” If they are complete, they will be recorded as such, but if something is lacking, He will say: “Look and see if My slave has any voluntary prayers to his credit.” If this proves to be the case, He will say: “Make up his (missing) obligatory prayers with his voluntary prayers.” Then the rest of his deeds will be dealt with in the same manner.’” (Saheeh al-Jaami’)

Q: If you do not pray salat out of laziness on purpose, are you a kafir or just a bad Muslim? Please answer.

A: Praise be to Allah(swt).
Imaam Ahmad said that the one who does not pray because of laziness is a kaafir. This is the more correct view and is that indicated by the evidence of the Book of Allaah and the Sunnah of His Messenger, and by the words of the Salaf and the proper understanding. (Al-Sharh al-Mumti’ ‘ala Zaad al-Mustanqi’, 2/26).

Anyone who examines the texts of the Qur’aan and Sunnah will see that they indicate that the one who neglects the prayer is guilty of Kufr Akbar (major kufr) which puts him beyond the pale of Islam. 

Among the evidence to be found in the Qur’aan is:

The aayah (interpretation of the meaning):

“But if they repent [by rejecting Shirk (polytheism) and accept Islamic Monotheism], perform As-Salaat (Iqaamat-as-Salaat) and give Zakaat, then they are your brethren in religion.” [al-Tawbah 9:11]

The evidence derived from this aayah is that Allaah defined three things that the Mushrikeen have to do in order to eliminate the differences between them us: they should repent from shirk, they
should perform prayer, and they should pay zakaat. If they repent from shirk but they do not perform the prayer or pay zakaat, then they are not our brethren in faith; if they perform the prayer but do not pay zakaat, then they are not our brethren in faith. Brotherhood in religion cannot be effaced except when a person goes out of the religion completely; it cannot be effaced by fisq (immoral conduct) or lesser types of kufr.

Also see a lot of related questions in the vast database at : Islamic FAQ's site.