"Looking at Ali is ibadat" (which is the 15th hadith about Imam Ali radiyallahu anh in this book) he says

: "Tabarani and Hakim narrated this with a hasan sanad from Ibn Mas'ud."


2 - The Prophet (Sallal L aahu Ta'ala Alayhi Wa Sallam) said: "To look at the face of Ali (Alayhis Salaam) is worship." 

(Al-Mustadrak, III, 152-153)

Details on the "SAHIH" hadith

Imam al Hakim (rah) made a whole chapter in his Mustadrak ala Sahihayn as: 

النظر إلى علي عبادة 

Translation: Looking at Ali is Ibadah [Mustadrak al Hakim (3/151)]

Then he brought this hadith: 

حدّثنا دعلج بن أحمد السجزي ، ثنا علي بن عبد العزيز بن معاوية ، ثنا إبراهيم بن إسحاق الجعفي ، ثنا عبد الله بن عبد ربّه العجلي ، ثنا شعبة ، عن قتادة ، عن حميد بن عبد الرحمن ، عن أبي سعيد الخدري ، عن عمران بن حصين قال: قال رسول الله : «النَّظَرُ إِلى عَلِيّ عِبادَةٌ». 

Translation (read chain above): Narrated by Abu Sa'eed al-Khudri who narrates from Imran bin Hussain (ra) that the Prophet (Peace be upon him) said: Looking at Ali (ra) is Ibadah. 

Imam al-Hakim (rah) said after narrating it: 

هذا حديث صحيح الإسناد وشواهده عن عبد الله بن مسعود صحيحة. 

Translation: This hadith has "SAHIH CHAIN" and the witness upon it is the report of Abdullah Ibn Masud (ra) which is (also) "SAHIH"[Mustadrak (1/152)]

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The Hadith is Hasan/Sahih. Imam Ibn Hajr al Makki (rah) said in his famous Sawaiq al Muhriqa which is written on refutation of Shi'ites:

أخرج الطبراني والحاكم عن ابن مسعود رضي الله عنه أن النبي قال النظر إلى علي عبادة إسناده حسن
Translation: It is narrated by Imam at-Tabrani and Imam al-Hakim from Ibn Masud (ra) that the Prophet (Peace be upon him) said: Looking at face of Ali (ra) is Ibadah. "THE CHAIN OF THIS REPORT IS HASAN" [As-Sawaiq al Muhriqa ala Ahl al Rafdh wal-Dhalaal (2/360)]