Akbar as we all know, is one of the greatest Moghal Emperors. He was a lover of mankind and respected the great and pious souls of all religions.
He had heard of Guru Nanak's reputation and his attempts to unite the Hindus and the Muslims. He desired to welcome him and honour him in his court. So he sent word to him through his minister, paying his respects and requesting him to grace his court. Guru Nanak replied to the minister: "I shall only respond to the call of God, the Emperor of Emperors and shall enter only His court."
The minister conveyed this message to the Emperor. Akbar's respect for Guru Nanak increased and so he sent word again to meet him at the mosque at least. Nanak consented and did come to the mosque at the appointed hour. Both Akbar and Nanak were welcomed by the mullah with due honour. According to the custom, the mullah should say the prayers first. So he sat on his knees and prayed loudly. Nanak laughed loudly. All the muslims in the temple got angry but dared not say anything because of the Emperor's presence. Then Akbar sat on his knees and prayed. Nanak at once laughed even more loudly. The atmosphere in the mosque was becoming tense. The faces of the devotees became red and their lips twitched to pounce upon Nanak. Akbar controlled them by way of silent gesture. Both of them came out. Akbar questioned Nanak with all humility: "Oh revered one!, may I know why you laughed loudly during the prayer session? Does it become you?"
Guru Nanak replied: "Oh king, how could I withhold my laughter when I could see clearly that neither the mullah nor your majesty where thinking of God while praying. The mullah was thinking of his ailing son and you were thinking of the pair of beautiful Arabian horses that were gifted to you. Is it worthy of either the mullah or your majesty to call that prayer? Is it not hypocrisy? The mullah and emperor sought pardon from Nanak and thanked him for opening their eyes to their own weakness.
Remember that prayer is not just a string of words of praise to God to be recited mechanically. It is an earnest attempt to awaken and arouse the divinity in us. We should say prayers with full concentration. What matters is the feeling, not either the voice or words. "Mere adulation is poor adoration".
Source: Chinna Katha II, 145