المُؤمِنُ بَيْنَ مَخَافَتَيْنِ: ذَنْبٌ قَدْ مَضَى لَا يَدْرِي مَا صُنْعُ اللهِ فِيه،ِ وَعُمْرٌ قَدْ بَقِيَ لَا يَدرِي مَا يَكْتَسِبُ فِيهِ مِنَ الْمَهَالِكِ
Imam Ṣādiq (Ýa) has stated: A faithful person is always worried about two things. The first includes the past sins for which he does not know how God will act in regards to them, while the second is for what is left of his life, during which he is uncertain of how he will behave.
The clearest sign of faith is a sense of responsibility towards one’s actions, as well as the duties that one must abide by. Someone who senses these two responsibilities will always be thinking of ways to fulfill them as well as making up for past lapses. Such a person will always have this thought in mind of how they can best act in light of their duties and how they can make use of the time that they have left.
 Uṣūl al-Kāfī, vol. 2, p. 7.