Disencumber yourselves of all attachment to this world and the vanities thereof. Beware that ye approach them not, inasmuch as they prompt you to walk after your own lusts and covetous desires, and hinder you from entering the straight and glorious Path.

Bahá’u’lláh, Gleanings from the Writings of Bahá’u’lláh, p. 275

Attachment to the earthly world, in relation to attachment to the spiritual world, is considered as a sin.

‘Abdu’l-Bahá, Some Answered Questions, p. 125

For attachment to the world has become the cause of the bondage of spirits, and this bondage is identical with sin.

‘Abdu’l-Bahá, Some Answered Questions, p. 123

For in the tavern of the mortal world the bile of the man of God is not removed. He will not rest a moment here and will not stain himself with the attachments of the world.

‘Abdu’l-Bahá, Tablets of ‘Abdu’l-Bahá v2, p. 380

God has given man a heart and the heart must have some attachment. We have proved that nothing is completely worthy of our heart's devotion save reality, for all else is destined to perish. Therefore the heart is never at rest and never finds real joy and happiness until it attaches itself to the eternal. How foolish the bird that builds its nest in a tree that may perish when it could build its nest in an ever-verdant garden of paradise.

‘Abdu’l-Bahá, Divine Philosophy, p. 136-137

It is because of this attachment that men have been deprived of essential spirituality and exalted position.

‘Abdu’l-Bahá, Some Answered Questions, p. 123