There are several matches for "certain" on 100 Philosophical Quotes from Bertrand Russell. Here are the most relevant ones:
“Not to be absolutely certain is, I think, one of the essential things in rationality.”
“A religious creed differs from a scientific theory in claiming to embody eternal and absolutely certain truth, whereas science is always tentative, expecting that modification in its present theories will sooner or later be found necessary, and aware that its method is one which is logically incapable of arriving at a complete and final demonstration.”
“Dogmatism and skepticism are both, in a sense, absolute philosophies; one is certain of knowing, the other of not knowing. What philosophy should dissipate is certainty, whether of knowledge or ignorance.”
“To teach how to live without certainty, and yet without being paralyzed by hesitation, is perhaps the chief thing that philosophy, in our age, can still do for those who study it.”
“Philosophy, though unable to tell us with certainty what is the true answer to the doubts which it raises, is able to suggest many possibilities which enlarge our thoughts and free them from the tyranny of custom.”
None are footnoted. They do tend to belie the same essence.
Many of the Google results on "nobody can be certain of anything" are Miss Rand's usage. Citing from Ayn Rand is dead, which has a few footnotes I didn't pursue:
"Despite the quotation marks, she is the actual author. It is a false, incorrect paraphrase of Hume’s Problem of Induction plus an anachronistic paraphrase of Bertrand Russell."
Once “There are no absolutes,” they chatter, blanking out the fact that they are uttering an absolute." is grasped, the rest is dotting i's and crossing t's.