Shahr bin Haushab reported: I asked Umm Salamah (May Allah be pleased with her), "O Mother of the Believers! Which supplication did the Messenger of Allah (PBUH) make frequently when he was in your house?'' She said: "He (PBUH) supplicated frequently: `Ya muqallibal-qulubi, thabbit qalbi `ala dinika (O Controller of the hearts make my heart steadfast in Your religion).''
[At-Tirmidhi].

Commentary: Steadfastness in religion is the conduct of great men, which cannot be acquired without the special Grace of Allah. There occur many turns in a person's life when he becomes slack or negligent in religious matters or when he deviates from the course prescribed by religion. This prayer has outstanding importance for such people who are faced with such a situation. They should recite this prayer very frequently. In fact they should make its recitation a permanent feature.