Thou knowest that in my love for Thee I have not sought any rest, that in proclaiming Thy Cause I have denied myself every manner of tranquillity, and that in the observance of whatever Thou hast prescribed in Thy Tablets I have not delayed to do Thy bidding. I have, for this reason, suffered what no man among all the inhabitants of Thy realm hath suffered.

Bahá’u’lláh, Prayers and Meditations by Bahá’u’lláh, p. 151

Do not delay. Undertake soon that which thou art intending.

‘Abdu’l-Bahá, Tablets of ‘Abdu’l-Bahá v3, p. 564

If, however, by delay and postponement they mean this, that in each generation only one minute section of the necessary reforms should be attended to, this is nothing but lethargy and inertia, and no results would be forthcoming from such a procedure, except the endless repetition of idle words. If haste is harmful, inertness and indolence are a thousand times worse. A middle course is best, as it is written: "It is incumbent upon  you to do good between the two evils," this referring to the mean between the two extremes.

‘Abdu’l-Bahá, The Secret of Divine Civilization, p. 108

Therefore, you must without delay employ your powers in spreading the effulgent glow of the love of God and so order your lives that you may be known and seen as examples of its radiance.

‘Abdu’l-Bahá, The Promulgation of Universal Peace, p. 8

There is so much suffering, such a great and desperate need for a true remedy and the Bahá’ís should realize their sacred obligation is to deliver the message to their fellowmen at once, and on as large a scale as possible. If they fail to do so, they are really partly responsible for prolonging the agony of humanity.

Shoghi Effendi, Lights of Guidance, p. 127