This point has been brought to a conclusion in the next verse by saying يُسَبِّحُونَ اللَّيْلَ وَالنَّهَارَ لَا يَفْتُرُونَ , (They proclaim His purity night and day, never slackening - 21:20).
Sayyidna ` Abdullah Ibn Harith ؓ عنہ said that he asked Ka` b al-Abbar ؓ how it was that angels kept reciting tasbih (proclaiming Allah's purity) all the time. Did they not have anything else to do? And if they did how could they do both things simultaneously, that is reciting tasbih and do other things. To this Ka'b replied "0 my nephew! Does performance of any of your jobs prevent you from breathing?" The truth is that tasbih comes to angels just as breathing comes to human beings which continues without stopping whatever else he may be doing.(Qurtubi, Al-Bahr ul-Muhit)