The Bodhisatta was once a teacher. One of his students mastered the three Vedas and could repeat them beautifully word for word by memory. Eventually he married and settled down. Later, the Bodhisatta visited his former student and asked if he still had mastery of all the verses. The student said that after he started a family, his mind became clouded and he could no longer repeat them. The Bodhisatta told him a clouded mind is like muddy water; things cannot be seen clearly through them.
In the Lifetime of the Buddha
The student was an earlier birth of a brahmin who had mastered the three Vedas and used to teach them to others. When he got married and settled down into family life and was preoccupied with wealth, livestock, and children, he could no longer reliably repeat the verses by memory.
One time he took some flowers and perfumes to give to the Buddha and sat down to talk with him. After he told the Buddha that he could no longer remember the verses word for word, the Buddha said this is what happens when your mind is obscured by passions and desires. He told the brahmin this story so he knew that the same thing had happened to him in the past.