“The rarest thing in this world is sincerity. How hard I work to clear showing off from my heart — and it seems to grow back in another colour.”

Yusuf ibn al-Husayn al-Razi (d. 304 AH / 916 CE) was a teacher of the spiritual path in Rayy. Ibn Rajab quotes this in his commentary on the first hadith of an-Nawawi’s Forty, “actions are only by intentions,” straight after Sahl at-Tustari’s remark that nothing is harder on the self than sincerity, “because it has no share in it.” It is the honesty of the second half that has kept it quoted.