1535-1545, Lectures on Genesis 48:21, WA 44, p. 720, ll. 28 ff.: "we encourage fearful hearts in this manner: 'Believe that you have been baptized into Christ. I absolve you from your sins in the name of Christ, who died for you and rose again, and said: "Because I live, you will live also"'" (LW 8, trans. Paul D. Pahl, p. 189).
1542 February 18, TR 5658a, WA TR 5, p. 295, ll. 27-30: "Alioquin illae cogitationes sunt diabolicae de praedestinatione. Ficht dich die cogitation an, so sprich: Ego sum filius Dei, sum baptizatus, credo in Iesum Christum pro me crucifixum, Iaß mich zu friden, du Teufel! Tum illa cogitation te deseret." This one is not included in LW 54.
In any case, the "inkwell" story is a myth. Cf.
- Oberman, Heiko Augustinus. "Luther against the devil." Christian century 107, no. 3 (January 24, 1990): 75-79, which refers to “later” references to the inkwell story separately from the following Luther-quotation (which I have not yet tracked to source): "'The only way to drive away the Devil is through faith in Christ, by saying: ‘I have been baptized, I am a Christian’” (see p. 105 (?) of the Oberman biography (Luther: man between God and the devil) from which this is excerpted.
- Hendrix, Scott A. http://www.christianitytoday.com/history/issues/issue-34/legends-about-luther.html, which calls the “inkwell” story in particular a kind of true myth.