He was asked:

What was the Prophet’s practice regarding combining prayers (peace and blessings be upon him)?

He replied:

As for combining prayers, he did so only on certain occasions, when pressing on with his journey and when he had a valid excuse under Islamic law. Thus, he combined prayers at Arafah and Muzdalifah, and sometimes during the expedition to Tabūk. If he set out before the sun passed its zenith, he would delay Zuhr until the time of Asr and then offer both together. This is established in the Sahīh. If he set out after the sun passed its zenith, it is reported that he would offer Zuhr and Asr together, as he did at Arafah. This is well known in the Sunan collections. This applied when he would not halt until the time of Maghrib, just as at Arafah he would not depart until sunset. If, however, he halted at the time of Asr, he would offer it at its appointed time.

Shortening prayers is therefore not like combining them. Shortening is a regularly observed Sunnah, whereas combining is an occasional concession. Any layperson who treats combining and shortening prayers as equivalent is ignorant of the Sunnah of the Messenger of Allah (peace and blessings be upon him) and of the statements of Muslim scholars. For the Sunnah of the Messenger of Allah (peace and blessings be upon him) distinguishes between the two, and the scholars