From the Bible:
In the Gospel of St. Matthew, Chapters 7 and 21, we read:
Ask, and it shall be given you; seek, and ye shall find; knock, and it shall be opened unto you. . . .
Verily I say unto you, if you have faith, and doubt not…if ye shall say unto this mountain, Be thou removed, and be thou cast into the sea; it shall be done.
And all things, whatsoever ye shall ask in prayer, believing, ye shall receive.
From the Bhagavad Gita, in the sixth Chapter:
Whenever the mind, fickle and restless, wanders off from its concentration, let the meditating yogi withdraw it resolutely, spurning every distraction (no matter how alluring!), and bring it back again and again under the control of the Self.