Bahá’u’lláh wrote:

“Know ye not why We created you all from the same dust? That no one should exalt himself over the other. Ponder at all times in your hearts how ye were created. Since We have created you all from one same substance it is incumbent on you to be even as one soul, to walk with the same feet, eat with the same mouth and dwell in the same land, that from your inmost being, by your deeds and actions, the signs of oneness and the essence of detachment may be made manifest.”
(The Hidden Words of Bahá’u’lláh, Part I – From the Arabic, #68)

‘Abdu’l-Bahá wrote:

“Consider the flowers of a garden. Though differing in kind, color, form, and shape, yet, inasmuch as they are refreshed by the waters of one spring, revived by the breath of one wind, invigorated by the rays of one sun, this diversity increaseth their charm, and addeth unto their beauty. How unpleasing to the eye if all the flowers and plants, the leaves and blossoms, the fruits, the branches and the trees of that garden were all of the same shape and color! Diversity of hues, form and shape, enricheth and adorneth the garden, and heighteneth the effect thereof. In like manner, when divers shades of thought, temperament and character, are brought together under the power and influence of one central agency, the beauty and glory of human perfection will be revealed and made manifest. Naught but the celestial potency of the Word of God, which ruleth and transcendeth the realities of all things, is capable of harmonizing the divergent thoughts, sentiments, ideas, and convictions of the children of men.

Therefore, the believers of God…must become the cause of the promotion of heavenly teachings and the establishment of the oneness of humanity…”
(Tablets of the Divine Plan, p. 94-95)

‘Abdu’l-Bahá said:

“…In the sight of God there is no distinction between whites and blacks; all are as one. Anyone whose heart is pure is dear to God — whether white or black, red or yellow. Among the animals colors exist. The doves are white, black, red, blue; but notwithstanding this diversity of color they flock together in unity, happiness and fellowship, making no distinction among themselves, for they are all doves. Man is intelligent and thoughtful, endowed with powers of mind. Why, then, should he be influenced by distinction of color or race, since all belong to one human family?”
(The Promulgation of Universal Peace, p. 425)

“God, the Almighty, has created all mankind from the dust of the earth. He has fashioned them all from the same elements; they are descended from the same race and live upon the same globe. He has created them to dwell beneath the one heaven. As members of the human family and His children He has endowed them with equal susceptibilities. He maintains, protects and is kind to all. He has made no distinction in mercies and graces among His children. With impartial love and wisdom He has sent forth His Prophets and divine teachings. His teachings are the means of establishing union and fellowship among mankind and awakening love and kindness in human hearts. He proclaims the oneness of the kingdom of humanity…”
(The Promulgation of Universal Peace, p. 297)