In the Visnu Purana it is written:

sa hanis tan mahac chidram
sa mohah sa ca vibhramah
yan-muhurtam ksanam vapi
vasudevam na cintayet

"If even for a moment remembrance of Vasudeva, the Supreme Personality of Godhead, is missed, that is the greatest loss, that is the greatest illusion, and that is the greatest anomaly."

(Quoted in Srila Prabhupada's purport to SB 2.9.36)