( Story told by Swami Vivekananda )
That drill business in the temples and churches — kneeling down at a certain time, standing at ease, and all that drill nonsense, all mechanical, with the mind thinking of something else — all this has nothing to do with real religion.
There was a great prophet in India, Guru Nânak, born [some] four hundred years ago. Some of you have heard of the Sikhs — the fighting people. Guru Nanak was [the founder and also] a follower of the Sikh religion.
One day he went to the Mohammedans’ mosque. These Mohammedans are feared in their own country, just as in a Christian country no one dare say anything against their religion. . . . So Guru Nanak went in and there was a big mosque, and the Mohammedans were standing in prayer. They stand in lines: they kneel down, stand up, and repeat certain words at the same times, and one fellow leads. So Guru Nanak went there. And when the mullah was saying “In the name of the most merciful and kind God, Teacher of all teachers”, Guru Nanak began to smile. He says, “Look at that hypocrite”. The mullah got into a passion. “Why do you smile?”
“Because you are not praying, my friend. That is why I am smiling.”
“Certainly not. There is no prayer in you.”
The mullah was very angry, and he went and laid a complaint before a magistrate and said, “This heathen rascal dares to come to our mosque and smiles at us when we are praying. The only punishment is instant death. Kill him”.
Guru Nanak was brought before the magistrate and asked why he smiled.
“Because he was not praying.”
“What was he doing?” the magistrate asked.
“I will tell you what he was doing if you will bring him before me.”
The magistrate ordered the mullah to be brought. And when he came, the magistrate said, “Here is the mullah. [Now] explain why you laughed when he was praying”.
Guru Nanak said, “Give the mullah a piece of the Koran [to swear on]. [In the mosque] when he was saying ‘Allah, Allah’, he was thinking of some chicken he had left at home”.
The poor mullah was confounded. He was a little more sincere than the others, and he confessed he was thinking of the chicken, and so they let the Sikh go. “And”, said the magistrate [to the mullah], “don’t go to the mosque again. It is better not to go at all than to commit blasphemy there and hypocrisy. Do not go when you do not feel like praying. Do not be like a hypocrite, and do not think of the chicken and say the name of the Most Merciful and Blissful God”.
Source: The Complete Works of Swami Vivekananda/Volume 9/Lectures and Discourses/Bhakti-Yoga