Source, in full
Though Stein famously used these words among some of her poems and writings, including Four Saints in Three Acts (1927) where they are lyrics to a hymn of communion, the phrase itself is far older and was published among anonymous "Love Posies" in 1596. It was a phrase often used in short letters, and also seems to have become incorporated into common epitaphs, one for Sarah Bryan in 1803 including it
Notes from Wikiquote
When I am dead and in my grave
And all my bones are rotten,
When this you see, remember me,
Let me not be forgotten.