Musannaf Ibn Abi Shayba 3:259

Yahya ibn Sa'id narrated to us, from Muhammad ibn 'Ajlan, from al-A'raj, from Abu Hurayrah, who said: "If you bow while the Imam is bowing, do not bow until you have taken your place in the row." Abu Bakr said: "If he and another person are there, he should bow behind the row. And if he is alone, he should not bow."