I haven't been able to find a reference to that direct quote. There are a number of things that mean the same thing. O'Sensei appears to have spoken often about being on a quest to find the perfect Budo, and that Aikido (or Aiki Budo) is the result of that search.
The closest quote I've found is:
"On reflection, Aikido can be seen as the root source of Japanese Budo." O'Sensei Memoirs, pg 18
I found several quotes which speak to that idea but are not as short.
“True budo is a work of love...Aikido is the realization of love.” ref
"“Understand Aikido first as budo and then as the way of service to construct the World Family." ref
Here's a Q/A session with O'Sensei about True Budo
First I learned Tenshinyo-ryu Jujitsu from Tozawa Tokusaburo Sensei, then Kito-ryu, Yagyu Ryu, Aioi-ryu, Shinkage-ryu, all of them Jujitsu forms. However, I thought there might be a true form of Budo elsewhere. I tried Hozoin-ryu Sojitsu and Kendo. But all of these arts are concerned with one-to-one combat forms and they could not satisfy me. So I visited many parts of the country seeking the Way and training. . . but all in vain.
A: Is that the ascetic training of the warrior?
O Sensei: Yes, the search for the true Budo. When I used to go to other schools I would never challenge the Sensei of the dojo. An individual in charge of a dojo is burdened with many things, so it is very hard for him to display his true ability. I would pay him the proper respects and learn from him. If I judged myself superior, I would again pay him my respects and return home.
B: When did Aikido come into being?
O Sensei: As I said before, I went to many places seeking the true Budo... Then, when I was about 30 years old, I settled in Hokkaido. On one occasion, while staying at Hisada Inn in Engaru, Kitami Province, I met a certain Takeda Sokaku Sensei of the Aizu clan. He taught Daito-ryu Jujitsu. During the 30 days in which I learned from him I felt something like an inspiration. Later, I invited this teacher to my home and together with 15 or 16 of my employees became a student seeking the essence of Budo. ref