SHAFAQNA – Abdullah ibn Sanaan who was a close companion of Imam Sadeq (AS) narrated: Imam Sadeq (AS) said: At the time of occultation (of Imam Mahdi (AJ)), when you cannot see Imam (AS), sometimes you may have doubts in your mind about Imam (AS). I will teach you a short Dua to repel these doubts. Then, Imam Sadeq (AS) said: Recite “Ya Allah, Ya Rahman, Ya Raheem, Ya Moqalebal Quloob, Thabbet Qalbi Ala Dinek.” Meaning, “O’ Allah (SWT), O’ the most merciful, O’ the most kind, O’ changer of the hearts, make firm my heart upon your religion,” .
 Beharul Anwaar, Vol. 52, Page 148.