< By Time > That is, the ages during which the deeds – both good and bad – of the sons of Aadam `alayhis salam take place. Maalik said that it refers to the time of `asr prayer, but the first explanation is more correct. Allah swears by it (time) that mankind is in loss, that is misfortune and ruin.
< Except those who believe and do righteous deeds > Allah excludes those of mankind who believe sincerely with all their hearts and do good deeds with their hands.
< And exhort one another to Truth > That is, in the face of misfortune or calamity, the things which have been written for us and the persecution of those who would seek to harm whoever orders the good and forbids the evil. It is reported that Musailamah (he was an arch-liar who lived at the time of the Messenger of Allah sallallahu `alaihi wa sallam and falsely claimed to be a brother-prophet) was visited by `Amr ibn al-`Aas after the Messenger of Allah sallallahu `alaihi wa sallam had begun his mission, but before `Amr had embraced Islaam and Musailamah said to him: "What does your companion (the Prophet sallallahu `alaihi wa sallam) say about this time?" `Amr replied: "A soorah has been revealed to him which is concise, yet elegant." "And what is it?" inquired Musailamah. "It is:
< By time, Verily, man is in loss, except those who believe and do righteous deeds and exhort one another to Truth and exhort one another to patience >." Musailamah thought about this for a while and then he said: "It has been revealed to me something like that." "What would that be?" asked `Amr. He replied, "Oh, wabr (Hyrax, an animal described by ibn Katheer as resembling a cat, having long ears, and a large breast and being for the rest, very ugly.)! Oh, wabr! You are merely two ears and a breast and the rest of you is but a hole." Then he asked: "What do you think, oh, `Amr?" `Amr replied, "By Allah! Surely you are aware that I know you lie!" Musailamah intended by the composition of these nonsensical verses to produce something which was the very antithesis of the clarity and conciseness of the Qur’aan; as a result it did not even convince a pagan idol-worshipper of the time.According to at-Tabaraani, whenever any two Companions of the Prophet sallallahu `alaihi wa sallam met, they would not part, until one of them had recited to the other Soorah Al-`Asr and then delivered Salaams upon him. Ash-Shaafi`I said: "If the people were to ponder on this Soorah, it would be sufficient for them." This is the end of the tafseer of Soorah al-`Asr, all praise is due to Allah and from Him proceeds all Grace.
Tafseer ibn Katheer - Juz' `Amma