Widmung ("Devotion") is one of many lieder (German art songs) that Schumann wrote out of the fullness of his love for his beloved Clara. He had met her when she was a young teen, and the two nurtured a growing romance over the next several years despite the objections and outright bitter legal battles with her father. When they finally married in 1840 (once Clara was of legal age), Schumann composed a great deal of romantic lieder describing his feelings for his wife.
I have sung this lied myself, and it is a beautiful, passionate song. The lyrics are a true declaration of love for your partner, and would be a wonderful addition to a classical-music-themed wedding ceremony. While it is (naturally) a vocal piece, it can be performed as a piano solo (one arranged by Schumann's friend and composer Franz Liszt and another by Clara herself), or even with the melody played by a violin and accompanied by piano.
Free sheet music (piano solo - Clara Schumann version) - includes other Opus 25 transcriptions done by Clara.
Sheet music (piano solo) - Schumann/Liszt version, hard copy
Du meine Seele, du mein Herz,
You my soul, you my heart,