Musannaf Ibn Abi Shayba 21:601

Abu Khalid al-Ahmar narrated to us, from Usamah, from Aban ibn Salih, from Mujahid, from Ibn Abbas, who said: "Indeed, Allah has angels who are extra, other than the guardians (Hafazah), who write down what falls from the leaves of the trees. So if one of you experiences a limp (or difficulty) while traveling, let him call out: 'Help, O servants of Allah, may Allah have mercy on you.'"