We used to speak during the prayer, one of us would address his brother about something he needed, until this verse was revealed, "Guard strictly the prayers, especially the middle prayer, and stand before Allah devoutly obedient" [Baqarah 2:238]. So we were commanded to remain silent and forbidden from speaking.
كنا نتكلم في الصلاة، يكلم أحدنا أخاه في حاجته، حتى نزلت هذه الآية: {حفظوا على الصلوات والصلوة الوسطى وقوموا لله قنتين} فأمرنا بالسكوت، ونهينا عن الكلام.
Bukhari narrated it in the tafsir (4534), and Muslim in the Masajid (35:539), both from the hadith of Ismail bin Abu Khalid, from Harith bin Shabil, from Abu Amr Shaybani, from Zayd bin Arqam, who mentioned it. Their wordings are identical, except that Bukhari did not mention, "and we were forbidden to speak."Qunut means worship and obedience. It is related from Abu Saeed, as a saying going back to the Prophet, "Every word in the Quran in which qunut is mentioned means obedience." Ahmad narrated it (11711), but it contains Ibn Lahia, about whom there is well known criticism, and his teacher Darraj narrated from Abu Haytham, and there is weakness in that chain. There are other hadiths on this same meaning, see the book of prayer.