‘Abdullâh b. Shaqîq Al-‘Uqaylî – Allâh have mercy on him – said:

The Companions of Allâh’s Messenger – Allah’s praise and peace be upon him – did not consider the leaving of any action to be kufr (unbelief), except the prayer (salâh).

Al-Tirmidhî, Al-Sunan hadîth 2622. Graded sahîh by Al-Albânî in his edition of Sunan Al-Tirmidhî.