Charles Spurgeon Commentary Matthew 6:7

"And in praying use not vain repetitions, as the Gentiles do: for they think that they shall be heard for their much speaking." — Matthew 6:7 (ASV)

But thou, when thou prayest, enter into thy closet, and when thou hast shut thy door, pray to thy Father which is in secret; and thy Father which seeth in secret shall reward thee openly. But when ye pray, use not vain repetitions, as the heathen do: for they think they shall be heard for their much speaking.

It is not very easy to repeat the same words often without it becoming a vain repetition. A repetition, however, is not forbidden, but a "vain" repetition. And how greatly do they err who measure prayers by the yard. They think they have prayed so much because they have prayed so long, whereas it is the work of the heart—the true pouring out of the desire before God—that is the thing to be looked at. Quality not quantity: truth, not length. Oftentimes the shortest prayers have the most prayer in them.