Uh....ok bet.
In season 3 ep 6, Walt and Gale are discussing poetry by Walt Whitman:
When I Heard the Learn'd Astronomer - Breaking Bad (orig: Walt Whitman) - YouTube
The very next scene after that, Walt is seen reading that book, "Leaves of Grass":
You never see Gale give it to him, but he did.
Now, fast forward to Season 4. Hank found the lab notes that Gale wrote. In Season 4 ep 4 he was showing them to Walt:
Breaking Bad 4x04 'Bullet points' You got me - YouTube
The front of the lab notes said:
Walt was sitting there like

because his initials are W.W.
But he thinks fast and says

"oh nah, it actually stands for Walt Whitman". Hank is like

Now fast forward
again to season 5. "Leaves of Grass" pops up in eps 3, 4, and 8 before Hank finds it. Walt just had it laying around the house.
In the back of the book it says:
So W.W. = Walter White. Meaning he was with Gale in the lab. Meaning he's Heisenberg.
Fred.