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It would be somewhat overstating the matter to say there is a heavenly mother “doctrine,” since all we know about a heavenly mother is that such a person exists. There is no reference to her in any LDS scripture. In fact, I know of only three places in official Mormon publications where this teaching can be found. It can be found in the hymn O My Father, a statement by the First Presidency entitled “The Family: A Proclamation to the World,” and a First Presidency statement from 1909.
The hymn O My Father reads,
In the heav’ns are parents single? No, the thought makes reason stare! Truth is reason; truth eternal tells me I’ve a mother there. When I leave this frail existence, When I lay this mortal by, Father, Mother, may I meet you in your royal courts on high? Then, at length, when I’ve completed all you sent me forth to do, With you mutual approbation Let me come and dwell with you. (Hymn 292)