Chometz-Free Only With Siyata Dishmaya

איתא במשנת חסידים (ניסן ג, ד): הנזהר ממשהו חמץ בפסח מובטח לו שלא יחטא כל השנה

In Mishnas Chassidim it is written: One who is careful [lit. one who is kept] from even the smallest amount of chometz on Pesach is assured that he will not sin the entire year.

The Chernobyler Maggid (quoted in Yalkut Meorei Ohr) pointed out that it should have said הזהיר (ha'zahir), meaning "one who is careful", and not הנזהר (ha'nizhar), "one who was kept", the implication of which is that he was kept from chometz by a third party. So he says that the truth is that since chometz is forbidden even as a ma'shehu (a smallest amount), it is impossible for a person to truly be sure to get rid of ALL of his chometz. Rather, after all the hishtadlus and work that we do to remove the chometz, one must ask from the Ribono shel Olam that He assist him that he should be able to be saved and kept from chometz b'mashehu. The only way to be careful from chometz entirely is by the intervention and assistance of Hashem so it is correct in saying "one who was kept from chometz".