Is dhikr a cause for the tranquility of the heart?

Q: Three years ago, I complained to one of the righteous men among us about my constant fluctuation between worldly matters and my lack of tranquility regarding my worship, such as fasting and prayer. I have been fasting and praying for ten years, and the temptations of this world are great. This man said to me: "Follow this method, perhaps your heart will find peace. Say: 'O Allah, send blessings upon our master Muhammad and upon his family and his companions,' one hundred times. Say: 'I seek forgiveness from Allah, there is no god but He, the Ever-Living, the Sustainer,' one hundred times. And say: 'There is no god but Allah alone, with no partner. His is the dominion and His is the praise. He gives life and He causes death, and He is over all things competent,' one hundred times." Is this correct or not? And is this what is intended by the saying of Allah the Exalted: [Surat al-Ra'd: 28]?
A: There is no doubt that increasing the remembrance of Allah, seeking forgiveness, and sending blessings and peace upon the Messenger of Allah (peace and blessings be upon him) are among the greatest causes for the tranquility and comfort of hearts, for settling down to Allah (Glorified and Exalted is He) and finding delight in Him (Glorified and Exalted is He), and for the removal of alienation, fluctuation, and confusion. So this man who advised you has done well in this advice. However, there is no fixed limit for seeking forgiveness, nor is there a fixed limit for sending blessings upon the Prophet (peace and blessings be upon him). Rather, it is prescribed that you increase in sending blessings and peace upon the Prophet (peace and blessings be upon him), and a specific number is not required. You should seek forgiveness frequently, whether one hundred times or more or less.
As for setting a limit of one hundred, there is no basis for it. However, you should increase your prayers upon the Prophet (peace and blessings be upon him), whether standing or sitting, at night and by day, on the road and at home; for Allah (Mighty and Majestic) said: [Al-Ahzab: 56]. And the Prophet (peace and blessings be upon him) said: So increase in this and be glad of good. There is no fixed limit; you may pray upon the Prophet as much as is easy for you, ten times or more or less, according to what is easy, without setting a specific number. Likewise, you should increase in seeking forgiveness, for you are commanded to do so. Allah (Glorified and Exalted is He) said: : [Al-Baqarah: 199]. And He (Glorified and Exalted is He) said: [Hud: 3].
So seeking forgiveness has a great status. In the authentic hadith, he (peace and blessings be upon him) said: . And it was narrated from him (peace and blessings be upon him) that he said: . So this has a great status. You should increase in seeking forgiveness at all times. And you should say after every obligatory prayer: "I seek forgiveness from Allah" three times, from the moment you say the taslim. And after that, you say: "O Allah, You are Peace and from You comes peace. Blessed are You, O Owner of Majesty and Honor." The Prophet (peace and blessings be upon him) used to begin with this when he said the taslim in his five prayers.
And you should increase in sending blessings and seeking forgiveness at night and day, at the beginning of the day and the beginning of the night, and at the end of the day. All of this is required.
As for the phrase "There is no god but Allah," the authentic hadith has come regarding it: . This hadith is recorded in the two Sahihs from Abu Hurayrah (may Allah be pleased with him). You should be careful to maintain this every day.
It is also prescribed for every Muslim man and woman to increase in saying: "Glory be to Allah and His praise, Glory be to Allah the Magnificent," due to the saying of the Prophet (peace and blessings be upon him): , recorded by al-Bukhari and Muslim in their two Sahihs.
Likewise, it is recommended for the Muslim to say: "Glory be to Allah the Magnificent and His praise, as many as His creation. Glory be to Allah, as much as He is pleased. Glory be to Allah, as much as the weight of His Throne. Glory be to Allah, as much as the ink of His words," three times. This has a great status, for it is established from the Prophet (peace and blessings be upon him) that he entered upon his wife Juwayriyah one morning, while she was in her place of prayer after the Fajr prayer, and he said: .
Likewise, "Glory be to Allah," "Praise be to Allah," "There is no god but Allah," and "Allah is the Greatest" have a great status. The Prophet (peace and blessings be upon him) said: . And he (peace and blessings be upon him) said: .
And he (peace and blessings be upon him) also said: .
And he (peace and blessings be upon him) said to Abu Musa al-Ash'ari (may Allah be pleased with him): He said: "Yes, O Messenger of Allah." He said: .
So you should increase in these dhikr phrases by which hearts find tranquility and conditions become straight, along with increasing in righteous deeds and sincere repentance from all sins, along with the mindfulness of Allah, standing firm on His religion, and always being cautious of sins.
It is prescribed for every Muslim to increase in these dhikr phrases and in sending blessings and peace upon the Messenger of Allah (peace and blessings be upon him), for in that lies great reward, a good outcome, the rectification and expansion of the heart, and the removal of fluctuation and confusion. For Allah (Glorified and Exalted is He) has promised that to whoever stands firm on His command, hastens to His obedience, increases in His remembrance, and increases in sending blessings and peace upon His Messenger (peace and blessings be upon him).
May Allah grant all of us steadfastness, and protect us all from the whispers of Satan, and guide us all to His straight path. There is no power and no strength except with Allah.