Reference for Sanhedrin 125:10
ואפילו לרבנן והתניא אינו חייב אלא על דבר שיש בו מעשה כגון זיבוח וקיטור וניסוך והשתחואה ואמר ריש לקיש מאן תנא השתחואה ר' עקיבא היא דאמר לא בעינן מעשה מכלל דרבנן סברי בעינן מעשה
R. Nahman said in the name of Rabbah b. Abbuha in Rab's name: As soon as he said, 'Thou art my God', he is liable, [Liable] to what? If to execution, this is stated [already] in the Mishnah? — Hence it means liable to a sacrifice. Now, is this so even in the view of the Rabbis? But it has been taught: He [the idolator] is liable [to a sacrifice] only for that which entails an action, e.g., sacrificing, burning incense, making libations, and prostration. Whereon Resh Lakish observed: Which Tanna maintains that a sacrifice is due for prostration? R. Akiba, who rules that a deed entailing [much] action is unnecessary.<span class="x" onmousemove="('comment',' Even if little action is involved, as in, e.g., prostration, a sacrifice must be brought. The same will apply to a formal declaration of belief, in which the action is very slight. This excludes a mere mental affirmation. ');"><sup>7</sup></span>