The following sermon from Ali ibn Abi Talib can be found in Nahj-ul-Balagha as well as in Tarikh al-Tabari.
فإِنَّ الغَايَةَ أَمَامَكُمْ، وَإِنَّ وَرَاءَكُمُ السَّاعَةَ تَحْدُوكُمْ تَخَفَّفُوا تَلْحَقوا، فَإنَّمَا يُنْتَظَرُ بِأوَّلِكُمْ آخِرُكُمْ .
Your aim (reward or punishment) is before you. Behind your back is the hour (of resurrection) which is driving you on. Keep (yourself) light and overtake (the forward ones). Your last ones are being awaited by the first ones (who have preceded). [Sermon 21]
It is said that Ali's saying "Keep light and overtake" in this sermon is the shortest expression with the deepest meaning. As I understand the first part of it - "keep light" - obviously refering to reduce attachment to material world and restrain from accumulating items/wealth, I am not sure about the second part -"overtake"- and the last sentence of the sermon.
Answer with references would be much appreciated.