Musannaf Ibn Abi Shayba 3:1548

Abu Usama narrated to us, from Ibn ‘Awn, from Muhammad, who said: "Whoever recites while he is sitting, then he bows and prostrates while he is sitting. And whoever recites while he is standing, then he bows and prostrates while he is standing." And al-Hasan said: "He has the choice; whichever of those he wishes, he may do."