Thank you for contacting Toronto Master Gardeners.
It is difficult from your photo to give an exact identification but the leaf in the picture points to being a willow, Salix.
Willows are a huge family that is notoriously difficult to name specific hybrids and species. The trees inter pollinate and naturally create many beautiful hybrids. To get a more specific identification one needs to look at the branch structure of the tree, the mature bark, the flowers in the spring and the trees size.
If you keep watching the tree through the season’s you maybe able to narrow it down further. I am including a link to help you get started.