What did Abdullah ibn Ubayy ibn Salul conceal within himself, given that his disbelief was one of hypocrisy?

The disbelief of hypocrisy is the outward declaration of faith with the tongue, while concealing in the heart that which contradicts it, such as doubt, denial, disdain, or illness. This results from the heart being devoid of true faith or some of the obligatory actions of the heart. Allah Almighty described the hypocrites by saying: "And among the people are those who say, 'We believe in Allah and in the Last Day,' but they are not believers. They [think to] deceive Allah and those who believe, but they deceive not except themselves and perceive [it] not. In their hearts is a disease, so Allah has increased their disease; and for them is a painful punishment because they [habitually] used to lie."