Musannaf Ibn Abi Shayba 1:867

Mu'tamir ibn Sulayman narrated to us, from his father, from Muhammad ibn Sirin, that he was asked about a heavy or large woman who cannot reach her body with her hand during the ritual bath (ghusl) from major ritual impurity (janabah) or menstruation. He said: "We hope for the mercy of Allah, which is greater than that."