It is narrated from Mus'ab bin Sa'd (may Allah have mercy on him) that he said: I said to my respected father (may Allah be pleased with him): "Dear father, what is your view about this saying of Allah Almighty: 'Those who are heedless of their prayer' [Surah al-Ma'un: 5]? Does it refer to the person who talks to himself in the prayer?" He said: "No; who among us does not talk to himself in the prayer? (That is, who can keep his thoughts under control?) Rather, the heedlessness in 'sahun' means delaying the prayer from its time."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3162