Zikar-e-Qalbi which is also known as Zikar-e-khafi or silent zikr, is being practiced by Naqshbandi Sufi followers. This way of zikar, Dhikr , 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.
Hazrat Khowaja Pir Sain Muhammad Soomar < Mohee ual Qalibul Momimnin
Some Sufi orders stress and place extensive reliance upon Dhikr, and likewise in Qadri Al-Muntahi Sufi tariqa, which was originated by Riaz Ahmed Gohar Shahi. The basic idea in this practice is to visualize the name of God, Allah, as having been written on the disciple's heart.