Kaṭha Upaniṣad 22

22

tisro rātrīr yad avatsīr gṛhe me anaśnan brahmann atithir namasyaḥ /
namas te 'stu brahman svasti me 'stu tasmāt prati trīn varān vṛṇīṣva ///1,1.9 //

9. As you have lived here. Oh Brahman, a venerable guest in my house for three nights fasting, be my prostration to you, Oh Brahman, may good befall me. Therefore, ask three boons in return.