[The ruling on the two rakʿahs after Maghrib and the two of Fajr]

Answer (and Allah grants success): There is no doubt that the command regarding the two rakʿahs after Maghrib is for recommendation, and the prohibition against leaving them is for dislike—and likewise for the two of Fajr and the two after ẓuhr. The Prophet’s leaving the two after Maghrib and after ẓuhr at ʿArafāt and Muzdalifah was to prevent people from imagining they are obligatory during ḥajj—especially in light of his words, “Take your rites from me.” Hence it is not narrated that he (may Allah bless him and family and grant them peace) prayed the night prayer (nafl) at Muzdalifah, nor the witr, in the descriptions of his ḥajj as in Jābir’s long ḥadīth.
Accordingly, their omission at ʿArafāt and Muzdalifah is specific and outside the general purport of, “Do not leave them in travel or at home,” so there is no conflict between the general and the specific.
As for his leaving the two after ẓuhr and after Maghrib when he (may Allah bless him and family and grant them peace) combined in Madīnah, that may have been to show they are recommended (not required), distinguishing between what is obligatory and what is recommended.
For this same purpose he combined ẓuhr with ʿaṣr and Maghrib with ʿishāʾ—to demonstrate that strict timing is not obligatory.
It may also be that his leaving the two after ẓuhr and after Maghrib was to indicate that they are not recommended when combining, which is why he did not pray them at ʿArafāt or Muzdalifah.
Some scholars of the school indeed said the two after ẓuhr and after Maghrib are not recommended for one who combines two prayers; hence al-Azhār states: “Supererogatory prayer between them is valid.”
Thus, the recommendation of the two rakʿahs after Maghrib and after ẓuhr applies to one who separates the two prayers in town or on a journey; as for one who combines them, they are not recommended for him—whether combining on a journey or at home.
Accordingly, the general statement “Do not leave them in travel or at home” is specified for the case of one who combines the two prayers, whether resident or traveling; thus there is no contradiction among the reports.
If someone says: the evidence for performing the two rakʿahs is verbal, while the evidence for their omission is practical, and verbal proof is—without dispute—stronger than practical proof when they conflict.
We reply: One does not resort to weighing proofs except when they conflict in every respect. Here, the verbal proof is general, and the practical proof is specific; there is therefore no conflict, for we can act on the specific in what it covers, and on the general in what remains.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1