“If I can not do great things, I can do small things in a great way” was credited in 1902 to American theologian James Freeman Clarke (1810-1888). It’s not certain when Clarke said this. An 1880 book of Clarke’s lectures has:
 
“‘She hath done what she could.’ This is the essential thing. We are not bound to do great things, but only to do what we can. (...) What a change would take place in all our lives if we only made up our minds to do what we can every day; having faith that, if we do anything right, however small, God will help us to do more.”
 
American personal-success literature author Napoleon Hill (1883-1970) wrote, “If you cannot do great things yourself, remember that you may do small things in a great way.”
   
Mother Teresa (1910-1997) is credited with a version of the saying. The book Mother of the Motherless: A Short Sketch of the Life and Work of Mother Teresa (1975) quotes:
 
“I do not want you to make big things; big things we forget about it; but small things with great love.”
   
     
Wikipedia: James Freeman Clarke
James Freeman Clarke (April 4, 1810 – June 8, 1888), an American theologian and author.
   
Google Books
Self-Culture: Physical, Intellectual, Moral, and Spiritual:
A Course of Lectures
Fourth Edition
By James Freeman Clarke
Boston, MA: James R. Osgood and Company
1880
Pg. 436:
“She hath done what she could.” This is the essential thing. We are not bound to do great things, but only to do what we can.
(...)
What a change would take place in all our lives if we only made up our minds to do what we can every day; having faith that, if we do anything right, however small, God will help us to do more.
 
Google Books
Seeing and Being and Other Sermons
By John White Chadwick
Boston, MA: George H. Ellis
1893
Pg. 9:
There is no lack of opportunity for spiritual greatness. Great souls declare themselves most frequently by doing little things in a great way. There is a great way and a little way of doing almost everything that waits the pressure of men’s hands.
 
31 October 1902, Postville (IA) Review, pg. 2, col. 5:
If I can not do great things. I can do small things in a great way.—James Freeman Clarke.
 
2 July 1904, The Daily Picayune (New Orleans, LA), “Words of Encouragement,” pg. 9, col. 4:
I cannot do great things. I can do small things in a great way.—James Freeman Clarke.
 
Google News Archive
20 May 1909, Toledo (OH) Weekly Blade,  “Morning Thoughts,” pg. 6, col. 2:
If I cannot do great things, I can do small things in a great way.
 
28 October 1911, Watertown (NY) Daily Times, pg. 16, col. 3:
James Freeman Clarke
If I cannot do great things, I can do small things in a great way.
 
Google Books
The Law of Success in Sixteen Lessons
By Napoleon Hill
Meriden, CT: Ralston University Press
1928, 2000 (Other Google Books versions are here and here—ed.)
Pg. ?:
If you cannot do great things yourself, remember that you may do small things in a great way.
 
Google Books
Mother of the Motherless:
A Short Sketch of the Life and Work of Mother Teresa
By S. D’Cunha
Bangalore: St. Paul Publications
1975
Pg. 74:
“If you could take it in turns to go there instead of your going for a cinema; instead of smoking two packets of cigarettes, you smoke only one and bring the second packet to them; little things; I do not want you to make big things; big things we forget about it; but small things with great love; no need for all of you to go, only those who want to love need go.”
 
Google Books
The Miracle of Love:
Mother Teresa of Calculta, Her Missionaries of Charity and Her Co-Workers
By Kathryn Spink
San Francisco, CA: Harper & Row
1981
Pg. 13:
Workers have responded to the cry of need which resounds from every corner of the world by doing “small things with great love” and the result has been what Pope Paul VI envisaged as early as 1964 as a “universal mission of love” - a mission, the full dimensions of which are yet to be appreciated.
 
Google News Archive
18 February 1991, Waycross (GA) Journal-Herald, pg. P-8, col. 1:
She recalled the words of Mother Theresa, “I cannot do great things, but I can do small things with great love.”
   
Google Books
Inspiration:
Your Ultimate Calling
By Wayne W. Dyer
Carlsbad, CA: Hay House
2006
Pg. 76:
I also love this advice, which was tendered by one of my favorite teachers, Napoleon Hill: “If you can’t do great things, do small things in a great way. Don’t wait for great opportunities. Seize common, everyday ones and make them great.”