Forgetting Torah Learning at Birth

Question: I heard that there is a Jewish legend that a baby learns the entire Torah in the mother's womb, and then an angel hits it on the lips during birth causing it to forget everything. What's the source for this legend?

Answer: The source for this is the Talmud - Niddah 30b. Here is the original text:

למדין אותו כל התורה כולה וכו' וכיון שבא לאויר העולם בא מלאך וסטרו על פיו ומשכחו כל התורה כולה

In English translation (Soncino):

It is also taught all the Torah from beginning to end ... As soon as it, sees the light an angel approaches, slaps it on its mouth and causes it to forget all the Torah completely

This is also one of the reasons for the celebration of Shalom Zachor - a party which is held Friday night following the birth of a boy (Taz, Yoreh Deah 265:13).

(See also Aish, AskMoses and Kollel Iyun HaDaf)