Abu Bakr As-Siddīq (may Allah be pleased with him) had a slave who used to give him a portion of his daily income as the master's share. Abu Bakr used to eat the food bought with that share

This slave of Abu Bakr had an agreement with him to pay him a fixed levy everyday. One day, he offered him some food, and Abu Bakr ate from it. The slave then said to him, "Do you know what this is?" Abu Bakr said: "What is it?" He answered that it was a reward for soothsaying that he had done for a man before embracing Islam, and that he was not good at soothsaying but he deceived the man, who later met him and gave him a reward.

The reward for soothsaying is unlawful, whether the soothsayer is good or bad at soothsaying. The Prophet (may Allah's peace and blessings be upon him) forbade the fee of soothsaying in a Hadīth reported by Abu Mas‘ūd al-Ansāri (may Allah be pleased with him). So when the slave said that, Abu Bakr inserted his hand in his mouth and vomited everything he had eaten, so that he would not nourish himself with unlawful food.

That money was unlawful because it was paid in return for an unlawful act, as the fee for a prohibited act is also prohibited.