There is a story. A powerful asura (demon) was doing severe penances to please Lord Siva. The vibrations of his austerities reached Heaven and Lord Indra was disturbed. He feared that when Lord Siva appears, the demon may ask for his throne as a boon.
When Devarishi Narada visited him, Indra explained his predicament. Narada smiled and assured him that such an eventuality shall not arise. Reciting the name of God – Narayana, Narayana – he went to the asura and complimented him on his deep devotion to Lord Siva. While parting he gave him one advice, “When praying, please do not think of a monkey.” “Why should I?”, replied the asura.
Later, whenever he sat for prayers the form of a monkey appeared in his mind and he kept on fighting with the image, further reinforcing the form of the monkey. And his whole austerity met with failure.
Baldeo Sahai in Essence of Indian Thought