The Promised Messiah (as) states:
“Hazrat Imam Hussain (ra) once asked” Hazrat Ali (ra), “‘Do you love me?’ Hazrat Ali (ra) replied, ‘Yes.’ Hazrat Hussain (ra) was greatly astonished at this as to how one’s heart could instil the love for two things at the same time. Following this, Hazrat Imam Hussain (as) said, ‘Who will you love if you had to choose between the two?’ Hazrat Ali (ra) replied, ‘Allah.’”
(Malfuzat, Vol. 7, p. 57)
Hazrat Musleh-e-Maud (ra) has also mentioned this incident and states:
“Hazrat Hassan (ra) asked Hazrat Ali (ra), ‘Do you love me?’ Hazrat Ali (ra) replied, ‘Yes, I do.’ Following this, Hazrat Hassan (ra) asked, ‘Do you love God Almighty?’ Hazrat Ali (ra) replied, ‘Yes.’
“Upon this, Hazrat Hassan (ra) said, ‘Then you are committing a form of shirk [associating partners with God] as shirk is defined as associating one’s love with the love for God Almighty.’ Hazrat Ali (ra) replied, ‘Hassan, I am not committing shirk. There is no doubt that I love you, but when your love comes up against my love for God Almighty, I will immediately cast my love for you aside!’”
(Quroon-e-Ula ki Namwar Khawatin aur Sahabiyat ke Iman Afroz …, Anwar al-Ulum, Vol. 21)