Allama ibn Hazm Al Andalusi رحمہ اللہ said:
وليكثر من قول لا إله إلا الله، فإنها ألفاظ تتم بحركة اللسان دون حركة الشفتين فلا يشعر بذلك الجليس.
Increase in the Dhikr "Laa ilaaha illaAllaah" verily, these are words which are pronounced without the movement of the lips, so that the one sitting next to you is unaware that you are in Dhikr since he is unable to see you utter this statement.
[Kitab Ar Rasail Al Talkhees La ibn Hazm 3/150]
This means that the one sitting next to you will not know that you are in Dhikr as it is not visible to him. The Salaf tried to hide their worship out of their fear they had for showing off, this was from their level of ikhlaas.
