…Then they pray to God, becoming sincere [in their] faith in Him…He [Sahl] said:Sincerity (ikhlāṣ) is witnessing (mushāhada). The life of the heart is in two things: in its root (aṣl) it is faith (īmān), and in its branch (farʿ) it is sincerity. Sincerity is a matter of great importance and the one who possesses it is wary lest his sincerity should not prevail till death, for actions are [judged] on the ones that come last — Worship your Lord, until what is certain comes to you [15:99]. His words, Exalted is He: