First specific usage of "mother" in reference to a mother in heaven.
W.W. Phelps, "Come to Me," Times and Seasons 6 no. 1, January 15, 1845, 783
Come to me; here's the myst'ry that man hath not seen;
Here's our Father in heaven, and Mother, and Queen;
Here are worlds that have been, and worlds yet to be;
Here's eternity,—endless; amen: Come to me.