On the authority of Abu Ayyoob al-Ansaaree:
A man came to the Prophet and said “Oh Messenger of Allaah give me advice and summarize it”. The Prophet (sallallaahu alayhi wa sallam) said (what means) “When you stand to pray, pray as if it is your last prayer, don’t speak with that which you would apologize for tomorrow,and be hopeless for that which is in the hands of the people”
(Musnad Ahmad/Authenticated by al-Albaanee)
Shaykh Abdur-Rahmaan bin Naasir as-Si’dee (may Allaah have mercy upon him said about this narration): “These are three pieces of advice and how beautiful they are . If the servant holds on to it, his affairs would be complete and he will be successful.
The first piece of advice consist of completing the prayer and striving to perform it in the best manner.That is by him (the servant) taking account of himself for every prayer that he prays, also that he will complete all of that which is within the prayer from that which is obligatory and recommended. He actualizes the state of Ihsaan (perfection) in it which is the highest of states. That is by him standing in the prayer bringing to mind his standing in front of his Lord, and that he is addressing Him with that which he says from his recitation, words of remembrance and supplication. He humbles himself to Him in his standing, bowing, prostration, lowering of himself and rising up in the prayer. That which will aid him upon this noble goal is settling oneself upon that (which was mentioned) without wavering and no laziness of the heart.
He also brings to mind in every prayer that it is the last prayer as if he will pray no other prayer. From that which is known that the one who is bidding farewell strives with his utmost and exerts himself in his affair with all that is at his disposal. He does not cease to carry these beneficial meanings and strong means (of attainment) until the matter becomes easy for him and he becomes accustomed to that. The prayer (being performed) in this manner prevents the one who is praying from every repugnant characteristic and encourages him to observe every beautiful characteristic. That is due to the effects of the increase of faith, the light of the heart, its happiness, and complete desire for the good within himself.
As for the second advice: It is the preserving of the tongue and closely watching over it. Indeed upon the preservation of the tongue there is the pivot (of ones life) and it is the controlling of affair of the servant. Therefore when the servant controls his tongue he controls all of his limbs. However when his tongue controls him and doesn’t protect him from the harmful speech then indeed his affair(s) become disorderly in his religion and worldly life. Therefore he should not speak with any speech except that he knows its benefit in his religion and his worldly life. Every speech that possibly has in it something that the person will be criticized for or apologize for then let him leave it. That is because if he speaks with it his speech owns him and he becomes a prisoner of it. Or it is possible that it brings about a harm upon him and he wont be able to fend it off.
As for the third advice: It is settling oneself upon being connected to Allaah alone in the affairs of his lively hood and hereafter. He doesn’t ask anyone except for Allaah. Nor does he desire (anything) except for His bounty. He settles within himself the despair for that which is in the hands of the people (he doesn’t hope for getting from them what they have). Indeed giving up hope (for that which is in the hands of the people) is a protection. Whoever gives up hope for having something becomes independent of it. So just as he doesn’t ask with his tongue except Allaah, likewise he doesn’t connect his heart to anyone except for Allaah. As a result of that he truly remains a servant of Allaah, free from the servitude of the creation. He certainly has freed himself from their slavery and he has earned by way of that the honor and the nobility. For indeed the one who is attached to the creation earns the disgrace and the downfall according to the degree of his attachment to them (the creation) and Allaah knows best”.
Taken from Bah-ja-tu Quloob al-Abraar. Explanation of Hadeeth 74.