After working with oil, pastel, and watercolors, I have intentionally returned to working with graphite. It is the most challenging mode of expression to master

Wahbi al-Hariri-Rifai (1992), The Spiritual Edifices of Islam: Drawings by Wahbi Al-Hariri-Rifai - National Museum (Saudi Arabia) Exhibit Booklet - 2002, Washington, D.C.: GDG Exhibits Trust & National Museum (Saudi Arabia), OCLC:56990773, retrieved on 25 June 2013. Quoted from inside cover.