I heard that Avraham Avinu had a daughter named Bakol. But in the Biblical narrative, there is no mention of a daughter to Avraham, only a son, Yitzchak, whom we all know well.
Did Avraham really have a daughter named Bakol, if a daughter at all?
Bava Batra (16b) records a dispute (found earlier in Tosefta Kiddushin (5:17 in R. Lieberman's edition) regarding the interpretation of Genesis (24:1) which states that God blessed Abraham "bakol" (literally: with everything):
Rabbi Meir says: The blessing is that he did not have a daughter.
Rabbi Yehuda says: On the contrary, the blessing was that he had a daughter.
Others say: Abraham had a daughter and her name was Bakkol.
Rabbi Elazar HaModa’i says: Abraham our forefather was so knowledgeable in astrology [itztagninut] that all the kings of the East and the West would come early to his door due to his wisdom. This is the blessing of bakkol, that he possessed knowledge that everybody needed.
Rabbi Shimon ben Yoḥai says: A precious stone hung around the neck of Abraham our forefather; any sick person who looked at it would immediately be healed.
Alternatively, what is the blessing of bakkol? That Esau did not rebel in Abraham’s lifetime, that is to say, as long as Abraham lived Esau did not sin.
Alternatively, the blessing of bakkol is that Ishmael repented in Abraham’s lifetime.
One of the seven views is that he had a daughter named bakol, while according to other views, he had no daughters.
Needless to say, as with any Midrashic dispute, later commentators fall on both sides. Zohar Vayechi (219a), Sefer HaBahir (78) cited in Recanti to Genesis (3:20), and others cite the view that he had a daughter.
Most commentators make no mention of the Midrash that he had a daughter, and explain בכל as simply meaning much blessing. Furthermore it is rejected by Ibn Ezra (Genesis 24:1). He explains that were his daughter's name "bakol", the verse would have needed to say that Avraham was blessed "b'bakol" בבכל; with bakol; not "bakol":
ודרש שבתו היתה שמה בכל צריך להוסיף בי"ת משרת
This problem is solved by a 13th century Syrian work called Hemdat HaHemda (cited by R. Menahem Mendel Kasher in his Torah Sh'lema to Genesis (24:1) which states that his daughter's name was actually "Kol"; not "Bakol", unlike our version of the Talmud.
If anyone doubted Ibn Ezra's intent, note that Ramban, who most frequently quotes Rashi and Ibn Ezra, and who frequently attacks Ibn Ezra, defends the Talmudic view, and attacks the one who disagrees with the Talmudic view. Rabbi Menahem Tsvi Eisenstadt explains here that the one who disagrees with the Talmudic view, is Ibn Ezra. This is also R. Menahem Mendel Kasher's understanding of Ibn Ezra (and Ramban) here. Indeed, R. Yitshak of Akko paraphrases Ramban (here) and explicitly attacks Ibn Ezra by name for writing that according to the Midrash the verse should have said בבכל instead of בכל.
Sefer Hagimatrios from Rabbi Yehuda Hachasid 94 says that Bakol was the daughter of Avraham and Sarah and since he had a boy and a girl from Sarah he did the Mitzva of Peru U'revu with Sarah.
Rabbi Yonasan Eibshutz in Tiferes Yonasan Vayeira 21:4 says that when the Pasuk says וימל אברהם את יצחק בנו whenever the Torah says את it means that a twin was born.
"הא דכתיב 'בן שמונת ימים כאשר צוה אותו אלקים' הוא נתינת טעם לשמונה של יצחק, גם דכתיב 'את יצחק', וכל מקום שנאמר 'את' מורה כי תאומים נולדו, כמבואר בגמרא (עי' יבמות סב.) ובמדרש (ראה ב"ר פד, כא) בשבטים, ואמרו חז"ל 'בת היו לו לאברהם אבינו ובכל שמה', ולא נזכר בתורה לידתה, אבל נראה שהיא תאומת יצחק, וזהו: 'את יצחק',
SPECIAL ZOHAR PORTION - Vayechi (“And he lived”) (21) “A joyful mother of children”