Praise be to Allah.
The correct view is that when a person reaches Muzdalifah, he should pray as soon as he arrives, because this is what the Prophet (blessings and peace of Allah be upon him) did. But in the case of the Messenger (blessings and peace of Allah be upon him), he put the prayers together at the time of the later prayer because the distance was far and he was riding a camel; moreover, he dismounted en route, and urinated and did wudoo’. In such a case, a person will not reach Muzdalifah until after the time for ‘Ishaa’ has begun. That is why he put the prayers together at the time of the later prayer. But now if someone arrives before the time for ‘Ishaa’ begins, he should pray Maghrib, but do we tell him to pray Maghrib then wait for the time of ‘Ishaa’ to begin, or do we tell him to put the two prayers together? Nowadays, we think that it is kinder to tell a person to put ‘Ishaa’ together with Maghrib, because that will spare him from having to make the effort to find water, because he may invalidate his wudoo’ and need to renew it, but be unable to find water, and if he goes to find water he may become lost if he leaves his spot. So we say: it is easier for people to pray ‘Ishaa’ with Maghrib, even if they reach Muzdalifah at the time of Maghrib. Ibn Mas‘ood (may Allah be pleased with him), if he reached Muzdalifah before ‘Ishaa’, would pray Maghrib then call for his supper; he would eat his supper then instruct his mu’adhdhin to give the call to prayer, then he would pray ‘Ishaa’. This indicates that he (may Allah be pleased with him) thought that if a person reaches Muzdalifah at the time of Maghrib, he should pray Maghrib and not put ‘Ishaa’ together with it. End quote.
Majmoo‘ Fataawa Ibn ‘Uthaymeen (23/63).