Is Combining Prayers without Reason Permissible?
Question

 

When I was in middle school I was very careless about the prayers. I used not to offer some prayers, but I read in one of your Fatwas that the one who fails to pray out of laziness does not have to make them up. But at some other time I used to offer some of the prayers, joining them together and shortening them without any excuse.
Do I have to make up these prayers or is my repentance sufficient? Since I started high school, I have been praying regularly. I ask Allah to make me and you steadfast.
Praise be to Allah.Does not praying constitute disbelief?
Not praying at all constitutes disbelief that puts one beyond the pale of Islam, according to the correct scholarly opinion. 
With regard to the person who prays sometimes and does not pray at other times some of the scholars are of the view that he is also a disbeliever, which is the view narrated from a number of the Companions (may Allah be pleased with them), and is also the view cited in Fatwas of the Standing Committee for Issuing Fatwas which was headed by Shaykh `Abd Al-`Aziz ibn Baz (may Allah have mercy on him). 
Other scholars are of the view that he does not become a disbeliever unless he does not pray at all , such as Shaykh Al-Islam Ibn Taymiyah (may Allah have mercy on him) and Shaykh Ibn ‘Uthaymin (may Allah have mercy on him). 
Do you have to make up missed prayers?
The scholars differed concerning the one who deliberately does not pray (such as one who is too lazy to pray , and the like): does he have to make up the prayers , as is required of one who sleeps and misses a prayer or one who forgets, and is it in fact more appropriate that the one who does not pray without any excuse should be instructed to make up the prayer, rather than the one who does have an excuse, as is the view of the majority of scholars, and the four Madhhabs and others are agreed on this point? 
Or is it the case that he does not have to do that, and if he does make them up it will not benefit him, either because not praying constitutes disbelief and the prayer of the disbeliever does not benefit him so long as he remains a disbeliever, and he is not instructed to make up what he missed during the time when he was a disbeliever and apostate; or because every act of worship is connected to a particular time, and if a person does not do it at the proper time with no legitimate excuse, it will not be accepted from him, because the Prophet (blessings and peace of Allah be upon him) said: “Whoever does an action that is not part of this matter of ours will have it rejected” (Narrated by Muslim, 1718)? 
The ruling on shortening the prayer when not travelling is no different from the ruling on not praying at all. If a person offers a prayer but omits one Rak‘ah or prostration, or he omits one of its pillars or essential parts deliberately, his prayer is invalidated and he is like one who did not pray it at all. He is closer to toying with the rituals, and such a person is in grave danger, if Allah does not bestow His mercy upon him and bless him by enabling him to repent sincerely. 
It was narrated that Ibn ‘Abbas (may Allah be pleased with him) said: Allah enjoined the prayer on the lips of your Prophet (blessings and peace of Allah be upon him); four Rak‘ahs when not travelling, two Rak‘ahs when travelling, and one Rak‘ah at times of fear. (Narrated by Muslim, 687). 
Ibn Hazm (may Allah have mercy on him) said: “The number of Rak‘ahs does not change (when travelling) except in the case of Dhuhr, ‘Asr and ‘Isha’: they are four Rak‘ahs when not travelling, for both healthy and sick individuals, two Rak‘ahs when travelling and one Rak‘ah at times of fear. There is scholarly consensus on all of that, except with regard to whether these prayers may be offered with one Rak‘ah at times of fear – there is a difference of opinion concerning that.” (Al-Muhalla, 3/185) 
Can one combine prayers without reason?
It is not permissible to join two prayers together without an excuse; if a person joins prayers together without any excuse or legitimate justification for doing so, then he is sinning, because he is going against the Shar`i texts that regulate the prayer, such as the verse in which Allah, may He be Exalted, says (interpretation of the meaning): “Verily, the prayer is enjoined on the believers at fixed hours” [An-Nisa’ 4:103]. 
Another such text is the Hadith in which the Prophet (blessings and peace of Allah be upon him) said: “Jibril (peace be upon him) led me in prayer at the Ka`bah twice. He led me in praying Dhuhr when the sun had passed the meridian and the shadow was the length of a sandal-strap; then he led me in praying `Asr when the (length of a thing’s) shadow was equal to its height; then he led me in praying Maghrib at the time when the fasting person breaks his fast; then he led me in praying `Isha’ at the time when the red afterglow had disappeared; then he led me in praying Fajr at the time when food and drink become prohibited to the one who is fasting. Then the next day he led me in praying Dhuhr when the (length of a thing’s) shadow was equal to its height; then he led me in praying `Asr when the (length of a thing’s) shadow was equal to twice its height; then he led me in praying Maghrib at the time when the fasting person breaks his fast; then he led me in praying `Isha’ when one third of the night had passed; then he led me in praying Fajr when it had grown light. Then he turned to me and said: O Muhammad, these are the times at which the Prophets before you prayed, the time is between each of these two times (for each prayer).” (Narrated by Abu Dawood, 393 and At-Tirmidhi, 149; Al-Albani said: Its Isnad (chain of narration) is authentic, in Sahih Abu Dawud – Al-Umm, no. 417)
Ibn Qudamah (may Allah have mercy on him) said: “The Muslims unanimously agreed that the five daily prayers are to be offered at certain times which are well-known and strictly defined, and several authentic Hadiths have been narrated concerning that.” (Al-Mughni, 1/224) 
Once this is established, it is not permissible to join two prayers together unless there is a reason to do so, such as travel , rain , or sickness . If there is no reason for joining prayers together, then you should abide by the original timings for the prayers. (Al-Mughni by Ibn Qudamah, 2/60) 
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said: 
“Because the Prophet (blessings and peace of Allah be upon him) defined the timings of the prayer in detail, doing the prayer at any other time constitutes transgressing the limits set by Allah. “And whoever transgresses the limits ordained by Allah, then such are the Dhalimoon (wrong-doers)” [Al-Baqarah 2:229]. Whoever offers a prayer before its time, knowingly and deliberately, is sinning and has to repeat it, but if he did not do that knowingly and deliberately, then he is not sinning, but he does have to repeat it. This applies to joining two prayers together at the time of the earlier prayer with no legitimate reason; the prayer that was brought forward is not valid and he has to repeat it. 
If a person delays a prayer until after its time, knowingly and deliberately, with no excuse, he is sinning and his prayer is not accepted, according to the more correct scholarly opinion. This applies to joining two prayers together at the time of the later prayer with no legitimate reason. The prayer that was delayed will not be accepted, according to the more correct scholarly opinion. 
The Muslim has to fear Allah, may He be Exalted, and not be careless with regard to this extremely important matter.” (Majmu` Al-Fatawa, 15/387) 
What you must do now is repent to Allah sincerely from what you have done, and mend your ways in the future. You have to make yourself respect the issue of prayer, which is the greatest duty that Allah requires of His slaves. 
If you err on the side of caution and try hard to make up the prayers that you missed, especially those which you shortened or joined together when you were not travelling, with no legitimate excuse, that will be better for you and is more likely to ensure that you have discharged your obligations. 
You should also do a lot of supererogatory actions, as much as you can, especially supererogatory prayers. Allah, may He be Exalted, says (interpretation of the meaning):
“And perform the prayer (Iqamat-as-Salat), at the two ends of the day and in some hours of the night (i.e. the five compulsory Salat (prayers)). Verily, the good deeds remove the evil deeds (i.e. small sins). That is a reminder (an advice) for the mindful (those who accept advice).
And be patient; verily, Allah loses not the reward of the good-doers.” [Hud 11:114-115]. 
And Allah knows best.