(Repetitively saying ALLAH ALLAH in the heart, silently, without any movement of tongue)
Remembering Allah (doing zikr) whether it is done in heart (silently) or verbally, individually or in groups, its merits and importance are well known. But from the Holy Quran and Hadith we find out that the merits of doing it silently in heart are many times more than doing it verbally. However if Allah Almighty specially bless someone and choose him as his lucky slave and bless him so that the person simultaneously practice verbal remembrance (dhikr) and his heart also practices in the same way; and he is promoted from verbal remembrance to heartily remembrance (zikr-e-qalbi); to such an extent that even if he is silent, tongue is quite, still his heart will not remain silent! This is called abundant remembrance, an ample rich remembrance (zikr-e-kaseer).
Hadhrat Mulla Ali Qari Rahmatulla Alleyhey who holds international fame and is a researcher and hadith master (muhaddith), has quoted the following hadith to show the high status of remembrance:
Abu’d-Darda’ reported that the Messenger of Allah (may peace be upon him) said, “Shall I inform you of the best of your actions and the purest of your property and the highest of your degrees and what is better for you than spending gold and silver and better for you than encountering the enemy and striking their necks and their striking your necks?” They said, “Yes, indeed!” He said, “Remembrance of Allah Almighty.” Mu’adh ibn Jabal said, “There is nothing which saves from the punishment of Allah more than remembrance of Allah.”
Mishkawt ul Massabih Page 198, Ahmad, Ibn Abi’d-Dunya, at-Tirmidhi and Ibn Majah
Commenting on this blessed hadith, the same great hadith master Hadhrat Mulla Ali Qari Rahmatulla Alleyhey says:
“This remembrance (zikr) means remembrance in ones heart. This is the same remembrance which has greater virtues than sacrificing lives and wealth. Because this is a veiled and hidden action of heart which is harder to perform by the self (nafs / one’s lowly desire). This is the greater struggle (Jihad-e-Akbar)!
Mirqat al-Mafatih, Page 22 Volume 3