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Zikar-e-Qalbi which is also known as Zikar-e-khafi or silent zikr, is being practiced by NaqshbandiSufi followers. This way of zikar, Dhikr ذکر, focuses on remembering Allah in one's heart. One has to feel that his heart is saying Allah, Allah, Allah, all the time day or night, standing or sitting, talking or while doing any work.
References
Remembering Allah (The only God), An Excerpt from Jalwagah-e-Dost (جلؤہ گاہ دوست), "Zikr: Jamaat Islah-ul-Muslimeen". Archived from the original on June 1, 2009. Retrieved April 17, 2009.