Shaikh Ibn ‘Uthaimeen said:
Humility of the heart is in greater need of being observed than humility of the limbs. That is because the one who has awe of Allah in his heart is [consequently] more observant of Allah The Mighty and Majestic and of His rights.
Hence, it is necessary for humility of the heart to be paid attention to more than humility of the limbs.It is possible for any individual to practise humility of the limbs, even inside his house. Everyone is able to stand praying motionlessly [out of apparent humility], however having a heart that is heedless. Therefore, it [the heart] is the basis. It is that that requires a person to pay attention to, taking constant and absolute care of it.
Tafsir Surah Yasin, p.36.