It is believed that Mendelssohn wrote this piece for cello and piano, titled Song Without Words, sometime around 1845. It was published after Mendelssohn’s death and given the designation Op. 109. The sheet music may be found here.
It was with great delight that I found this video of cellist Jacqueline du Pré performing Op. 109 (Iris du Pré, piano). I listened to other performances, but I kept coming back to this one. Perhaps it was the richness and sweetness of tone, or the expressiveness of the playing. I hope you will enjoy it as well.