First, I disagree with Yeqon. If he did present a resignation, it sounds pretty interesting. In my mind, it would be merely a gesture, since he knew that it wouldn't be accepted, but it still speaks a lot about political stratagems and protocol, and the circumstances for each attempt at resignation should also be an indication of the seriousness of a certain setback or of the need to reassert leadership.
Now, as to the question at hand. I remember reading in this board that Stalin presented his resignation when Lenin's much debated "last will" was presented.
Googling around, the answer came up on Ludo Marten's Another view of Stalin, where the author directly quotes Stalin's "The Troskyist Opposition, Before and Now.
Stalin wrote: `It is said in that ``will'' Comrade Lenin suggested to the congress that in view of Stalin's ``rudeness'' it should consider the question of putting another comrade in Stalin's place as General Secretary. That is quite true. Yes, comrades, I am rude to those who grossly and perfidiously wreck and split the Party. I have never concealed this and do not conceal it now .... At the very first meeting of the plenum of the Central Committee after the Thirteenth Congress I asked the plenum of the Central Committee to release me from my duties as General Secretary. The congress discussed this question. It was discussed by each delegation separately, and all the delegations unanimously, including Trotsky, Kamenev and Zinoviev, obliged Stalin to remain at his post ....
A year later I again put in a request to the plenum to release me, but I was obliged to remain at my post.
So that's twice. It's not too farfetched to think he again has resorted to this, as a way of obtaining a vote of confidence.