Bismillaah Al-Hamdulillaah wa salatu wa salaamu 'ala rasulullaah
Amma-ba'd
Ikhlaas in the statement of al-Ikhlaas
Ibn Hazm رحمه الله تعالى mentioned a tremendous benefit in his book 'Takhlis li wujuh At Takhalis' while mentioning actions which are easy to perform but carry much reward.
و لـيـكـثـر مـن قـولِ(لا إلـه إلَّا الله) فـإنَّهـا ألـفـاظٌ تـتِـمُّ بـحـركـةِ الـلـسـانِ دونَ حـركـةِالـشَّـفـتـيـن فـلا يَـشـعُـرُ بـذلـكَ الـجـلـيـسُ ....
The person should pronounce the statement La-illaaha ilAllaah (Nothing has the right to be worshipped except for Allah) many times, because verily these are words which can be pronounced completely with the movement of the tongue without moving the lips, thus those sitting with you will not be aware that you are saying it.
Source: http://www.sahab.net/forums/index.php?showtopic=114406
Translated by: Rasheed ibn Estes Barbee (hafidhahullaah)