Musnad Ahmad 1:79

Hashim ibn al-Qasim narrated to us, he said: Isa—meaning Ibn al-Musayyib—narrated to us, from Qays ibn Abi Hazim, who said: I was sitting with Abu Bakr al-Siddiq, the Caliph of the Messenger of Allah, may Allah bless him and grant him peace, one month after the death of the Prophet, may Allah bless him and grant him peace. He mentioned a story, and it was called out among the people: "The prayer is a gathering" (al-salatu jami'ah). This was the first prayer among the Muslims for which it was called out: "The prayer is a gathering." The people gathered, and he ascended the pulpit—a thing that had been made for him to deliver sermons upon—and this was the first sermon he delivered in Islam. He said: He praised Allah and extolled Him, then said: "O people, I truly wish that someone else had sufficed me in this matter. If you hold me to the Sunnah of your Prophet, may Allah bless him and grant him peace, I cannot bear it; for he was protected from Satan, and the revelation used to descend upon him from the heaven."