Luke 11:9-13:

And I say unto you, Ask, and it shall be given you; seek, and ye shall find; knock, and it shall be opened unto you.
For every one that asketh receiveth; and he that seeketh findeth; and to him that knocketh it shall be opened.
If a son shall ask bread of any of you that is a father, will he give him a stone? or if he ask a fish, will he for a fish give him a serpent?
Or if he shall ask an egg, will he offer him a scorpion?
If ye then, being evil, know how to give good gifts unto your children: how much more shall your heavenly Father give the Holy Spirit to them that ask him?

See also:

Healing and the Holy Spirit
How to be Baptized with the Holy Spirit
Objection: Jesus Literally Said to “Keep On Asking.”  Therefore, We Should Bug God Like the Widow in Jesus’ Parable.