What is the source for saying “God willing” before something one plans to do?

This source-based Torah study examines the question: What is the source for saying “God willing” before something one plans to do? The canonical Hebrew discussion develops the issue through the weekly Torah portion and the sources cited there, preserving the distinctions, tensions, and practical implications of the original text. This localized discovery layer does not add any new halakhic, medical, legal, scientific, historical, theological, or empirical ruling or claim beyond the canonical Hebrew source.