587 - Muhammad ibn Salamah and al-Harith ibn Miskin informed us—the latter reading it aloud while I listened, and the wording is his—from Ibn al-Qasim, who said: Malik narrated to me, from Abu al-Zubayr al-Makki, from Abu al-Tufayl 'Amir ibn Wathilah, that Mu'adh ibn Jabal told him that they went out with the Messenger of Allah, peace and blessings be upon him, in the year of Tabuk. The Messenger of Allah, peace and blessings be upon him, would combine the noon and afternoon prayers, and the sunset and night prayers. One day he delayed the prayer, then came out and prayed the noon and afternoon prayers together, then went in, then came out.

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Al-Bukhari also recorded it (1111), as did Ahmad (13799), Abu Dawud (1218), and Ibn Hibban (1592) via chains from al-Mufaddal ibn Fadalah, with the same report.
Muslim recorded it (704): (47), and Ibn Hibban (1456) via the route of Layth ibn Sa'd, from 'Uqayl ibn Khalid, with the same report, with the wording: "When the Prophet, peace and blessings be upon him, wanted to combine two prayers while traveling, he would delay the noon prayer until the beginning of the afternoon prayer's time, then combine them."
Al-Sindi said: His statement, "until the time of the afternoon prayer, then he came down and combined them," apparently indicates that he combined them during the time of the afternoon prayer. Those who do not hold this view interpret his statement "until the time of the afternoon prayer" to mean "until near the time of the afternoon prayer," and interpret the combining as combining in action rather than in time—that is, he prayed the noon prayer at the end of its time such that the exit of its time and the entry of the afternoon prayer's time were connected with his completion of it, then he prayed the afternoon prayer at the beginning of its time. And Allah Most High knows best.
Ahmad (12111) and Abu Dawud (1204) also recorded via the route of al-Mishaj ibn Musa al-Dabbi, who said: I said to Anas ibn Malik: Tell us what you heard from the Messenger of Allah, peace and blessings be upon him. He said: When we were with the Messenger of Allah, peace and blessings be upon him, on a journey, and we said, "The sun has declined" or "it has not yet declined," he would pray the noon prayer, then set off. This narration is understood as referring to hastening the prayer, not to performing it before its time or performing it while doubting the entry of its time, as noted in the margins of "al-Musnad" from al-Sindi.
The hadith has other chains; see the commentary on it in "al-Musnad" (13584).
It will also come via the route of Jabir ibn Isma'il, from 'Uqayl ibn Khalid, with the same report, at number (594), with the addition of combining the sunset and night prayers.
(1) Referenced in the margins of (K) and (YH) with a copy.
(2) In (R), (Q), and (M): "the noon prayer," which is a variant in the margins of (K), (H), and (YH).