Abu Ma'mar (may Allah have mercy on him) said: We asked Sayyiduna Khabbab (may Allah be pleased with him): How did you know of the recitation of the Messenger of Allah (peace and blessings be upon him) in the Zuhr and Asr prayers? He said: "By the movement of his jaws."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3674