STRENGTHENING ONES IMAN
There are many du'as, and each of them will help one to increase their iman.
The increase of iman comes with the fulfillment of Allah's obligations and recommendations and abstaining from his prohibitions. Visiting the Muslim graveyard and pondering over death, and what is to come after death increases one's faith.
The remembrance of Allah increases iman and causes hearts to attain tranquility. Allah Most High says:
"Indeed in the remembrance of Allah do hearts find tranquility" (13:28).
All the Qur'anic du'as and prophetic prayers help to make this happen. The best form of remembrance has been considered La ilaha illa 'Llah. Hence, an abundant recitation of this formula with concentration and devotion should have the desired effect, insha' Allah.
Al-Hizb al-Azam (The Great Litany) by Mulla Ali Al-Qari comprises of many duas from the Qur'an and hadith which are divided into seven sections to be recited throughout the week.
by Abdurrahman ibn Yusuf