Do you remember who else said that?--The Good Samaritan! It's from the parable that the Lord told about the poor man that was beaten and robbed along the road by thieves, and the Good Samaritan picked him up and took him to the inn and told the innkeeper, "Whatsoever thou spendest, I will repay thee." (Luk.10:30-37)
The Good Samaritan is really the Lord, and the innkeeper is His steward, like you or me!--And whatever we spend to help in His Work of rescuing lives and saving souls, He will more than repay! In fact, He has promised that if we obey Him and open the windows of our lives upon others, He "will open the windows of Heaven, and pour out such a blessing, there will not be room enough to hold it!" (Mal.3:10)
That's the way God is: As you pour out, He'll pour in, and He'll never let you outgive Him! He always gives you much, much more than you ever give! As long as you will continue to overflow on others, God will continue to overflow His blessings upon you! And the more you give, the more He'll give you back in every way!
Why don't you try it and see what a blessing you can be and how much God will bless you for it!