Burton, in "Elementary Number Theory", states that the following problems are due to Euler 1775:
If $n$ is a triangular number, then so are $9n+1$, $25n+3$ and $49n + 6$.
R. F. Jordan in the J. of Recreational Mathematics (1991, vol.23, p.78) proves the following generalization:
Let $t_k$ be the $k$-th triangular number. For all $k$ and all triangular numbers $n$, $(2k+1)^2n + t_k$ is also triangular.
and wonders whether Euler actually proved this generalization. Jordan wasn't the first to prove this, see for example the following proposed solution to the 49th Putnam 1988 question B6.
I looked through the Euler Archive, but didn't locate the suitable 1775 manuscript (either appearing in 1775, written in 1775, or presented in 1775).
What did Euler prove and where? What does Burton refer to?
EDIT: The only part of the question that remains unanswered is: As Euler didn't seem to prove this generalization, who was the first to notice it?