When praying to Hashem to save Sedom and Amorah, Avraham makes a plea to Hashem that if he can find fifty tzadikim, Hashem should save the city. Avraham responds to Hashem's granting of the request, הנה נא הואלתי לדבר אל ה' - may I begin to speak to my lord - ואנכי עפר ואפר - [though] I am [but] dust and ash (יח:כז). He then asks Hashem to spare the city for the sake of forty five tzadikim who may be living there. The בית הלוי points out that Avraham lowered himself in two ways by saying ואנכי עפר ואפר. עפר, dirt, never has shape and is never important except for growing plants. אפר, ash, previously had a shape and was significant but now can not do anything and is nothing. Avraham is saying he never was important, like dust, and certainly is not important now, like ashes.
This is why, according to the בית הלוי, the Gemara in Chulin states בשכר שאמר אברהם אנכי עפר ואפר זכה לעפר סוטה ואפר פרה - Once he said I am dust and ashes he merited to dust of סוטה and ashes of the red heifer. The עפר סוטה - dust from the Beit Hamikdash floor that the woman suspected of adultery consumes, tests whether she sinned or not, while the אפר פרה אדומה - the ashes of the red cow, rescues a person from impurity.
נחלת יעקב asks, why only by forty five does Avraham say ואנכי עפר ואפר? Rashi says that forty five is nine for each city and Hashem would combine with them in each city to make ten. The נחלת יעקב cites a Gemara from Sanhedrin (קה.) to teach us the lesson of Avraham's dialogue. It discusses a story told by Shmuel as an example of Bnei Yisrael's defiance of a Navi. Ten people came in front of יחזקאל who told them to do teshuva. They responded, "A slave sold by his master and a wife divorced by her husband each have no further relationship or connection with one another. So too, Bnei Yisrael have no further connection to Hashem." Hashem tells יחזקאל to respond איזה ספר כריתות - Where is the divorce document?" The Gemara quotes another Pasuk that says והעולה על רוחכם היו לא תהיה - What you have in mind will never come to pass. The נחלת יעקב asks, why does the Gemara need to quote two Pesukim if one would have been enough?
He answers that if just איזה ספר כריתות applied, Bnei Yisrael might never have thought that since there were ten, the שכינה is present and they are believed to say גרשתני - he divorced me - no further proof is needed. So when Hashem asks איזה ספר כריתות, Bnei Yisrael can respond we do not need the ספר כריתות since we have ten and can say גרשתני. Because of this, Hashem also says העלה על רוחכם - you cannot be haughty or גסי רוח and those who are גסי רוח - אין השכינה שורה בהם -the Shechina does not reside in them, so they are not נאמן (believed) to say גרשתני. Therefore, the question of Hashem to Bnei Yisrael, איזה ספר כריתות still holds. We see from this that אין שכינה שורה למטה except when the people are humble and שפל רוח.
That is why Avraham says ואנכי עפר ואפר only by forty five, which as Rashi states is nine. The Shechina resides with Avraham because Avraham says - ואנכי עפר ואפרI am שפל רוח so the Shechina resides, and will combine to make ten, as Rashi says. We see from Avraham that by being שפל רוח, we can be זוכה to have the Shechina reside amongst us.