In the Surah Al-Isra, verse 27, it is stated that, Surely squanderers are ever brothers of satans How should we understand this verse?

In the Quran, God orders humans to earn livelihood in Halal (Religiously permissible) ways, spend part of the substance to the poor and needy, look after the rights of kin and also emphasizes that He, Himself is the only true owner of the all possessions. Furthermore, He forbids dissipation, which is the scum of humankind, and describes the people who squander their wealth senselessly as brothers of Satan. He orders in the verse related with the subject as:

And give his due to the relative, as well as the destitute and the wayfarer; and do not squander (your wealth) senselessly. Surely squanderers are ever brothers of satans; and Satan is ever ungrateful to his Lord. (Al-Isra Surah, 17:26, 27)

Prophet Muhammad (pbuh) also forbade dissipation and announced humans to spend, as it is required in Quran, for they will be held responsible for that on Judgment Day. Concerning the point, he even informed humans about being thrifty about home furnishings and again forbade squandering for that. He orders in the hadith, which was narrated on the authority of Jabir as, One mattress is for man, one other is for his wife, third one is for the guest, but the fourth one is for the Satan. (1)

The fourth mattress mentioned in the hadith, was defined as a real one by some of the Islamic scholars and adduced as if Satan spends nights on that. However, some other scholars claimed, by the hadith, that it was intended to show Satans encouragement of dissipation. (2)

Sources:
(1) Müslim, Libas, 41; Abu Davud, Libas, 45/4142; Nesai, Nikah, 82/3383; Müsned, III, 293.
(2) Nevevi, Minhac, XIV, 59-60