The Big Bang Theory Season 5 Episode 3 : The Pulled Groin Extrapolation
Amy asks Leonard to accompany her to a friend's wedding as Sheldon was too immature at the last wedding they attended (and because he plays with model trains). At the wedding, the two bond over their mutual partners and Amy eventually convinces Leonard to dance. He eventually begins to get into it, but ends up pulling a muscle in his groin. As thanks, Leonard affectionately kisses her on the cheek, but Amy immediately misinterprets it as his falling in love with her, and immediately confides in Penny. The next morning, Leonard describes the evening to Sheldon, but inadvertently makes it sound like he and Amy had sex, prompting an unexpectedly jealous reaction from Sheldon, who hits him with his model train and tells him that she is "not for you!"