Midrash su Salmi 78:37

R. Simeon the son of Levi said: Moses learned it from the law concerning (a woman) who has been violated, as it is stated: Then the man that lay with her shall give unto the damsel’s father fifty shekels of silver (ibid., v. 29). Since we have violated the word that the Holy One, blessed be He, spoke to us: Thou shalt have no other gods before Me (Exod. 20:3), and erected the calf, let each of us give fifty pieces of silver. Others say he learned it from the law of enticement, as it is said: And if a man entice a virgin that is not betrothed, he shall surely pay dowry for her (Deut. 22:15). The dowry one pays for a virgin (who has been enticed) is fifty pieces of silver. Hence, since they enticed Him with their mouths and deceived Him with their tongues, For their heart was not steadfast with Him, neither were they faithful in His covenant (Ps. 78:37), let each one give fifty shekels of silver.