You should run the following commands to ensure which Python is being used
which python
/home/ubuntu/src/anaconda3/envs/fastai/bin/python
which jupyter
/home/ubuntu/src/anaconda3/envs/fastai/bin/jupyter
Then inside the notebook run this
import sys
sys.executable
This tells you which python is being used. Based on the path that you are showing it looks like Pythin 3.5 would run. You could run the jupyter notebook from the conda environment manually. So if your conda environment is named fastai then you could run something like
/home/ubuntu/src/anaconda3/envs/fastai/bin/jupyter
This should run the conda environment’s jupyter. If not, check whether you activated the conda environment by running something like conda activate fastai from the root of your conda environment.