Texas Guest- would you please post some lyrics from the recording you have? And by 'early last century,' what exactly do you mean? 1900-1920, 1920-1940 or later?

The phrase itself is little help since it has been used by many for a long time- there are European books and hymns from as early as the 17th c. that use that phrase.

There are Negro spirituals with that title:
De angels are watchin' obuh me. Ballanta-Taylor, St. Helena Island Spirituals.
Angels watchin' over me. Johnson, Utica jubilee singers spirituals.

Probably there are more modern gospel and spiritual songs with the same message.

More information please; someone here probably will be able to help.