The Holy Quran in Surah At-Tawbah (repentance), which was revealed in Medina, has invited the believers to two truths: the first one is the piety and the second one is a command to be with the truthful people (يَا أَيُّهَا الَّذِينَ آمَنُوا اتَّقُوا اللَّهَ وَكُونُوا مَعَ الصَّادِقِينَ- At-Tawbah). According to the noble narratives and prominent figures of Shi'i, such as Tabarsee, Allameh Helli, Allameh Majlisi and Muhammad Bāghir, the truthful people are those who are honest and truthful in their words, actions, intention and moods, and these four facts are only true for the Infallibles. According the verse in the Surah At-Tawbah, you will gain the prosperity both in this world and hereafter through being and accompanying the truthful people (the infallible Imams), then your prayers are answered, and you will have an easy death, purgatory and day of judgement.
source : erfan.ir