Immediately, ‘Abdu’l-Bahá slapped him...

One day in the birúní[outer section] of the blessed House of Haifa, He [‘Abdu’l-Bahá] stated, “I have forgotten my handkerchief.” One of the friends who was known as Ustád Muhammad-‘Alí Banná, remarked to this effect, “How is it possible for the manifestation of Truth to forget anything?” Immediately, ‘Abdu’l-Bahá slapped him and stated, “Over forgetting a handkerchief that man puts a hundred souls to test.”

(Ahang Rabbani, A Lifetime with `Abdu'l-Bahá: Reminiscences of Khalíl Shahídí, pp. 100-101)

https://www.scribd.com/document/2454317/A-Lifetime-with-Abdul-Baha