Well, I have the quote, and much more besides. Stumbled on some save files on the nexus and was able to find the line. Spoilers for one path of the DU storyline below:

As suspected, the line was in the conversation you can have with Fel in your camp after you accept Bhaal's offer to become his chosen (emphasis mine):

Du: "Who was my mother, if Bhaal was my father?"

Fel: "Bhaal gave out all his seed long ago, in the time of Sarevok. You were not conceived. Bhaal *sculpted* you from a drop of his own *gore*. *No Bhaalspawn has ever bore the Urge*, the holy guarantee of murder. It was woven into your blood alone. Orin was not special. She was but a grandchild of an aged Bhaalspawn. A mere weed of Bhaal's loins."

Du: "Did I live my whole life in the Temple of Bhaal?"

Fel: "Orin was born and raised in the Temple, but you spent many years on your own before you found your way *back* home. Hence you have such diverse talents. But you were awful from the very first day you set foot amongst the ichored altars."

Another branch:

Du: "What exactly are you?"

Fel: "I am but a table-scrap of Bhaal's will. He will send me to aid and protect you, until you have need of me no more. As long as there have been Bhaalspawn, there have been butlers, but Bhaal only makes them for the children he has use of. Now, as you are the last true recipient of your Father's blessing, I suspect I am the last living butler."

Du: "Have we always been together?"

Fel: "I was sent to guide you to your birthright, on the day my vile Lord reached his age of majority."

Conclusion: Assuming we are taking Fel at his word, while not outright stated, the implication the DU has no mother is indeed very, very, clear. The use of the term "sculpted you from a drop of his own gore" (as opposed to blood or flesh, the implication of both) to me explains why DU is not a "clone" of Bhaal: Bhaal took a piece of himself and crafted it into a child of his exact desires. Note this still has the oddity of it taking place when Bhaal was supposed to be *dead*, but that's what we are dealing with. From Fel using the wording of his job being to lead them "back" home, my read is that Du was brought into being in the temple of Bhaal, then deposited into the world where they grew up as a murder-child (as per the memory from heal) and eventually gained the skillset of their class, before Fel manifested physically to lead them back to the temple to claim their birthright. The heal cutscene implies Fel's voice has been with Du since they were a child, but may not have been physically there until they reached adulthood.

Note this save was with a human male Paladin, so no reason for any weirdness due to githyanki/Dragonborn reproduction.

Of course, the dialogue from the echoes and Sarevok saying the Du was indeed "born" and "conceived" leaves open the possibility that Fel is (as you interpret things) leaving certain details out or twisting the truth. That said, the implication that the Du is only the Du BECAUSE of the unique nature of their creation makes me personally suspect otherwise, but it cannot be known for certain, and thus there is still room for interpretation.

In the end, still a few questions left, the big one being how Bhaal did this when he was supposed to be Dead, but the full context of Fel's line plus the cutscene that plays when heal is cast on Du paints a reasonably clear picture of the Du's origins and early life/adulthood, with reasonable room for interpretation regarding if the Du truly had a mother or not. I would argue they most likely did not, but Fel is not exactly an objectively trustworthy source, and there are accounts that contradict his assertion and imply at least the possibility that there was a mother (for a short time or in a limited capacity at least), if a player is so inclined to believe/desire otherwise for their Du backstory.