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 says to his Lord. And do not raise your voices over one another in recitation during the prayer."
أن النبى 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.
Imam Ahmad narrated it (4928), and from him Tabarani in al-Kabir (12/428), from Ibrahim bin Khalid, who said, Rabah narrated to us, from Maamar, from Sadaqah al-Makki, from Abdullah bin Umar, and he related it. Its chain of transmission is sahih.Sadaqah al-Makki is Sadaqah bin Yasar al-Jazari al-Makki, who died at the beginning of the caliphate of Banu Abbas, which was in the year one hundred and thirty-two, and he is among the men of the Sahih.Imam Ahmad also narrated it (5349), as did Ibn Khuzayma (2237), both by way of Ibn Abu Layla, from Sadaqah al-Makki, from Ibn Umar, and he related the hadith.Ibn Abu Layla is Muhammad bin Abdul-Rahman bin Abu Layla, whose memory was poor, but the corroboration of Maamar, who is Ibn Rashid, confirms that he did not err in this hadith.On this subject there are other hadiths related by Ali, which I have mentioned in the Book of Prayer, so refer to it there.