Shaykh Saalih Al-Fawzan – May Allah preserve him – was asked about an individual who prays the five daily prayers, but not in the correct times and occasionally misses all of the prayers of a day and prays them all together later on.
The Shaykh said:
Safeguarding the performance of the (obligatory) prayers at their correct times with the congregation (for men) is obligatory for the Muslim, and it is not permissible to exit the prayer from its (legislated) time.
As Allah says:
إِنَّ الصَّلَاةَ كَانَتْ عَلَى الْمُؤْمِنِينَ كِتَابًا مَّوْقُوتًا
Verily, the prayer is enjoined on the believers at fixed hours.
[An-Nisaa Verse 103]
Meaning it is obligatory to perform them at specific times, and in a narration:
“Indeed there are actions (done) for Allah in the night that He does not accept in the day and actions in the day He does not accept in the night” (1)
And exiting the prayers from their correct times is neglecting them.
فَخَلَفَ مِن بَعْدِهِمْ خَلْفٌ أَضَاعُوا الصَّلَاةَ وَاتَّبَعُوا الشَّهَوَاتِ فَسَوْفَ يَلْقَوْنَ غَيًّا
إِلاَّ مَنْ تَابَ
Then, there has succeeded them a posterity who have given up As-Salat [i.e. made their Salat to be lost, either by not offering them or by not offering them perfectly or by not offering them in their proper fixed times, etc.] and have followed lusts. So they will be thrown in Hell.
Except those who repent…
[Soorah Maryam Verse 59-60]
And combining between two prayers – in the time of one of them ie Dhuhr with Asr or Maghrib with Isha – is not permissible except due to a legislated excuse. As for combining the prayers for a complete day then this is not permissible and the prayer is not correct in this manner.
Al muntaqa 5/56
(1) Advice of Abu Bakr to Umar whilst on his deathbed
See link for full narration