Musannaf Ibn Abi Shayba 3:6462

Al-Fadl ibn Dukayn narrated to us, from Abu Hilal, from Qatadah, from Umm al-Hasan, from Umm Salamah: That she used to say between the two rak'ahs or the two prostrations: "O Allah, forgive, have mercy, and guide to the most upright path."