Sheikh Muhammad Muhammad Sâlim `Abd al-Wadûd
A Muslim may combine – on account of hardship – the Zuhr and `Asr prayer at that time of either of those prayers, and likewise the Maghrib and `Ishâ' prayer at the time of either of those prayers.
Ibn `Abbâs narrated that the Prophet (peace be upon him) combined between Zuhr and `Asr and between Maghrib and `Ishâ’ without traveling or raining. [Sahîh Muslim]
It is known from the Sunan books that the Prophet (peace be upon him) allowed the woman who suffered from continuous bleeding to combing between such prayers. This was to make things less difficult for her.
Consequently, a Muslim may offer Zuhr and `Asr together at the time of Zuhr if he or she is reasonably certain that it will not be possible to offer the `Asr prayer during the time of `Asr without considerable hardship. Likewise, a Muslim may offer Maghrib and `Ishâ' together at the time of Magrib if he or she is reasonably certain that it will not be possible to offer the `Ishâ' prayer during the entire time of `Ishâ' without considerable hardship.
For the second prayer to be valid before its prescribed time, the person must have a reasonably certain expectation of hardship. When this is the case, the combination of the two prayers is valid and the obligation of both prayers is fulfilled by their observance.
If, thereafter, the reason for the hardship is unexpectedly removed, then the person does not have to make up the second prayer, since at the time it was offered, the conditions of validity were all met.
And Allah knows best.
Source: Islam Today