Musnad Ahmad 26:689

Yahya narrated to us, from Shu'bah, from al-Hakam, from Mujahid, from Abu Ma'mar, from Abdullah—he said: I heard him—once he attributed it [to the Prophet], then he omitted it—"He saw an amir or a man who performed two taslims, so he said: 'Where did you learn this?'"