Risālat al-Ḥuqūq
Hadith 45وحق المسؤول إن أعطى فاقبل منه بالشكر والمعرفة بفضلة، وإن منع فاقبل عذره.
The right of him from whom you ask is that you accept from him with gratitude and recognition of his bounty if he gives, and you accept his excuse if he withholds.