It is narrated from Sayyiduna Abu Ayyash al-Zuraqi (may Allah be pleased with him), who said: The Prophet (peace and blessings be upon him) led us in the Zuhr prayer at Usfan. When he (peace and blessings be upon him) finished, the idolaters, who were commanded by Khalid bin al-Walid, said: "They were in such a state that, had we wished, we could have caught them heedless and unawares and harmed them." So between Zuhr and Asr this verse was revealed concerning the prayer. Then the narrator related a hadith similar to it.
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5120