Describing complete piousness (asceticism or Zohd in Arabic) which is mentioned in Ayah 23 of Surah Al-Hadeed, Imam Ali (AS) and Imam Sadeq (AS) said:

All of Zohd are about two sentences in the holy Quran which say: Do not regret what you have lost, and do not get overjoyed for what has been given to you; and whoever does not regret for whatever has been lost, and does not get overjoyed for whatever has been lost; has attained Zohd completely [1].

[1] Nahjul Balaghah, Faizul Islam, Hekmat 431; Tafseer Qomi, Vol. 2, Page 146; Beharul Anwaar, Vol. 70, Page 52.

Source: en.shafaqna.com