In which episode of which season in "The Big Bang Theory" does Sheldon's mother sing the "Soft Kitty" song to him? I have yet to hear it!
In S05E06 The Rhinitis Revelation, Sheldon's mom, Mary, sings "Soft Kitty" to Sheldon after he gets sick from being in the rain after he felt that his mother was ignoring him over spending time with his friends
She used to sing in episode 11 of season one. This episode was called The Pancake Batter Anomaly. She used to sing to him especially when he was sick as a child.