Allah said:

﴿وَقُومُواْ لِلَّهِ قَـنِتِينَ﴾

(And stand before Allah with obedience.) meaning, with humbleness and humility before Him (i.e., during the prayer). This command indicates that it is not allowed to speak during the prayer, as speaking contradicts the nature of the prayer. This is why the Prophet refused to answer Ibn Mas`ud when he greeted him while he was praying and said afterwards:

«إِنَّ فِي الصَّلَاةِ لَشُغْلًا»

(The prayer makes one sufficiently busy.) (i.e., by the various actions of the body, tongue and heart involved during the prayer.)

Muslim reported that the Prophet said to Mu`awiyah bin Hakam As-Sulami when he spoke during the prayer:

«إِنَّ هذِهِ الصَّلَاةَ لَا يَصْلُحُ فِيهَا شَيْءٌ مِنْ كَلَامِ النَّاسِ، إنَّمَا هِي التَّسْبِيحُ وَالتَّكْبِيرُ وَذِكْرُ الله»

(The ordinary speech people indulge in is not appropriate during the prayer. The prayer involves only Tasbih (praising Allah), Takbir (saying Allahu Akbar, i.e., Allah is the Most Great) and remembering Allah.)

Imam Ahmad reported that Zayd bin Arqam said, "One used to address his friend about various affairs during the prayer. Then when this Ayah was revealed:

﴿وَقُومُواْ لِلَّهِ قَـنِتِينَ﴾

(And stand before Allah with obedience.) we were ordered to refrain from speaking.'' The Group (i. e., the Hadith collections), except Ibn Majah, reported this Hadith.