"One of the constant commandments that can be performed at any moment of one’s life and even by thought alone is ahavas Hashem, love of G-d." (source)

"Ahavas Yisrael is an indicator for ahavas HaShem, i.e., if a person is lacking in ahavas Yisrael, he is lacking in ahavas HaShem, for one who loves the father should surely love the children." (source)