Musannaf Ibn Abi Shayba 3:1038

Abu Khalid al-Ahmar narrated to us, from Ibn ‘Ajlan, from Sa‘id ibn Abi Sa‘id, from Abu Hurayrah, who said: Ka‘b ibn ‘Ujrah said to me: "When you enter the mosque, greet the Prophet, may Allah bless him and grant him peace, and say: 'O Allah, open for me the doors of Your mercy.' And when you leave, greet the Prophet, may Allah bless him and grant him peace, and say: 'O Allah, protect me from Satan.'"