He—may Allah have mercy on him—was asked:

About clearing the throat, coughing, blowing, groaning and similar actions during prayer: do they invalidate it or not? Which of these actions, or others, invalidate prayer? According to which school, and what is the evidence for that?

He replied:

All praise is due to Allah, Lord of all worlds.

The foundational principle in this matter is the Prophet’s statement—may Allah bless him and grant him peace:

“No ordinary human speech is permissible in this prayer of ours.”

He also said:

“Allah introduces such commands as He wills, and among the commands He has introduced is that you must not speak during prayer.”

Zayd ibn Arqam said: “We were commanded to remain silent and forbidden to speak.” This is a matter on which the Muslims agree.

Ibn al-Mundhir said: “The scholars unanimously agree that whoever speaks deliberately during his prayer, without intending to correct any aspect of it, has invalidated his prayer. A person speaks deliberately when he knows that he is praying and that speaking is prohibited.”

(I say: The scholars have differed concerning someone who speaks forgetfully, out of ignorance, under coercion, or for the benefit of the prayer. All these cases are disputed within Ahmad’s school and among other scholars.