Najaf is considered so sacred because it’s the site of the tomb of Ali, ʿAli b. AbiṬāleb, the first imam of the Shiites with graves attributed to Adam, Nuḥ (Noah). 

Muslim who revered Ali (a.s)  as Mohammed’s rightful heir became known as the Shi’at Ali, or “faction of Ali”—the forefathers of today’s 150 million Shiites.

In addition to Ali’s tomb, the city also boasts one of the world’s largest cemeteries, the Wadi-us-Salaam (“Valley of Peace”). Several prophets( Hod, Ṣāleḥ , etc. ) are buried there, and some believe that Ali himself endorsed the site as part of heaven. Shiites from around the world long to be buried there.

Muslim tradition held that Abraham—who is a venerated figure in all three of the world’s major monotheistic faiths—once visited the area, and he predicted that it would someday host a shrine of great importance. Abraham also stated that those buried in Najaf would be guaranteed entry to paradise. So Ali had requested that, when he died, he be buried not in his capital of Kufa but rather in neighboring Najaf.

Abraham wasn’t Najaf’s only famous ancient visitor. According to Muslim lore, one of Noah’s sons refused to board the Ark, choosing instead to sit atop a mountain that covered present-day Najaf.But the mountain crumbled, drowning the son, and a river appeared in its place. The river eventually dried up, giving the city its name —Najaf means “Dry River.”

Source: Shafaqna