Wa alaikum AsSalaam,
i've read articles and heard many lectures by scholars (Arab) but have'nt heard any such thing. everywhere, i heard that Malakul Maut comes when a man's soul is to be taken out i.e.,the moment of his death.
also read and heard that an angel(not Malakul Maut) comes to us in fajr and stays with us till asr. another angel comes in asr and stays with us till fajr. ALLAH asks each one what his servant was doing when they reached him and left him. and if a person is found praying the 2 prayers at their appropriate time then they'll reply that- when they reached him, he was praying and when they left him, he was praying. this pleases ALLAH.....................i am sorry, i dont know the exact words of the hadith but it's meaning is as mentioned above.
and ALLAH knows best....