The Prophet ﷺ performed itikaf and addressed the people, saying, "When one of you stands in prayer, he is conversing privately with his Lord, so let each of you be mindful of what he is saying to his Lord. And do not raise your voices over one another in recitation during the prayer."
حدثنا إبراهيم بن خالد حدثنا رباح عن معمر عن صدقة المكي
أن النبي ﷺ اعتكف وخطب الناس فقال أما إن أحدكم إذا قام في الصلاة فإنه يناجي ربه فليعلم أحدكم ما يناجي ربه ولا يجهر بعضكم على بعض بالقراءة في الصلاة
Its isnad is sahih. Sadaqah al-Makki is Sadaqah bin Yasar, whose reliability was already established at 4584. We preferred the view that he narrates from Ibn Umar, relying on this very isnad here, and we add further that Bukhari mentioned him in al-Kabir 2/2/294 and cited his narration from Zuhri from Ibn Umar of a hadith about ramal, then said, "Ibn Uyayna said, from Sadaqah, from Ibn Umar." So Sadaqah does narrate from Ibn Umar, and Sadaqah belongs to the generation of Zuhri, for he was a contemporary of Ibn Umar and met him.This hadith will come at greater length at 5349, 5585, and 6127, by the route of Ibn Abu Layla from Sadaqah from Ibn Umar. Hafiz Haythami quoted the longer narration in Majma al-Zawaid 2:265 and said, "Ahmad, Bazzar, and Tabarani in al-Kabir narrated it, and it contains Muhammad bin Abu Layla, about whom there is some discussion." It appears he did not come across this isnad through Maamar from Sadaqah, which does not contain Ibn Abu Layla.See also what has already passed in the Musnad of Ali at 763, 752, and 817, and in the Musnad of Ibn Masud at 4309.