Imam Fakhruddin Razi has written in Arbaeen that in the lifetime of the Prophet (S) a group of companions was famous for its austerity; like Abu Zar Ghiffari, Salman Farsi and Abu Darda etc. All these great personalities followed the example of Ali (a.s) in simplicity and austerity.
It is narrated from Qabida in Majmaul Ahbab fil Manaqib al Ashab that: We have not seen anyone among the people more pious than Ali (a.s).
Ibn Athir has narrated from Hasan bin Salih in Tarikh Kamil that once in the presence of Umar Ibn Abdul Aziz the conversation steered towards piety. He said: Of all the people, Ali (a.s) was the most pious.
It is mentioned in Usud al-Ghaba that Ammar Ibn Yasir has narrated that the Messenger of Allah (S) told Amir’ul-Mu’minin (a.s), “O Ali! The Almighty Allah has bestowed you with such a merit as He has not bestowed anyone else. And that is piety in the world, which is an ornament for the people in view of Allah. Allah has made you such that neither you got anything from the world nor the world got anything from you. He gave you the love for poor people and He made you satisfied by their following you and He made them pleased by making you their Imam.”
It is narrated from Amir’ul-Mu’minin (a.s) that he said, “The Messenger of Allah (S) told me, ‘O Ali! When people became attached to the material world and forsake the Hereafter. Usurp the inheritance of others and destroy the religion and plunder the wealth of Allah; what would be your condition?’ I said, ‘I would leave them and forsake whom they follow. And I would betake Allah, His Prophet and the abode of the Hereafter. I would be patient on worldly calamities and difficulties, till the time I meet you.’ The Prophet said, ‘It is true, you will do this only.’”
Ahmad Ibn Hanbal has written in his Manaqib that one day the Satan suggested to Ali (a.s), “Always keep the public treasury full of wealth and gold coins.” Ali (a.s) came to the Baitul Maal (Public treasury) and ordered that people may be summoned. Then he started distributing till everything was finished and he said, “O Gold and Silver, defraud others.” When the public treasury became empty he ordered it to be sprinkled with water. Then he performed two rakats (Units) of prayers of thankfulness.
It is mentioned in Usdul Ghabah that Imam Hasan (a.s) narrated, “Neither my respected father collected wealth nor he left behind anything, except for six hundred Dirhams with which he desired that slaves may be freed.” In the same book it is narrated from Abu Naeem that he heard Sufyan say, “The Amir (Ali) never placed a brick upon a brick or a bamboo upon a bamboo to construct a house. If he wanted he could have inhabited from Medina to Jurab.”
Reference :Akhlaq al-A’imma, Morals & Manners of the Holy Imams