Musnad Ahmad 27:1107

Yazid ibn Harun narrated to us, Sulayman al-Taymi informed us, from Abu Mijlaz, from Ibn Umar: "That the Prophet, peace and blessings of Allah be upon him, prostrated in the first rak'ah of the Dhuhr prayer." His companions thought that he had recited [the Surah] "Tanzil al-Sajdah." He [the narrator] said: "And I did not hear it from Abu Mijlaz."