It does not ruin your fast; however, you are guilty of committing a sinful act; so if you did resort to it, never fail to ask forgiveness of Allah. Among others, you may choose to freely use the following supplication:

Allaahumma innaka afuwwun tuhibbu al-‘afwa fa’fu annee

(O Allah, You are Forgiving; You love to forgive, and so forgive (my sins) for me). Here is another one:

Allaahumma ighfir lee dhanbee kullahu: diqqahu wa jillahu, amdahu wa khata’ahu, awwalahu wa aakhirahu, wa sirrahu wa alaaniyyatahu

(O Allah, forgive me all of my sins: major ones as well the minor; the one I have done deliberately as well as those I have done in ignorance; the first one as well as the last; the open one as well as the secret). You may read both du’as any time; it is especially good to read them during sujud (prostration) in Prayer.